Installing Windows after Ubuntu is not the recommended process for a dual boot Windows and Ubuntu system, but it is possible.

• First, you have to make sure that the 50GB free space is available, resizing your Ubuntu by using gParted if necessary.


• Also use gParted or Disk Utility to format the the free space as NTFS, this prevents Windows 7 from creating an additional boot partition in addition to the "C" drive.



• Next, boot to the Windows DVD installer and install Windows on the NTFS partition. Upon reboot, Windows will automatically boot and you won't see the grub menu allowing you to choose Ubuntu, because the Windows bootloader has replaced grub.




• Now, what you need to do is run Ubuntu from LiveCD or LiveUSB and install boot-repair.



• 
  

  To install boot-repair, open up a terminal by pressing Ctrl+Alt+T and type the following commands:

  
  
  

  sudo add-apt-repository ppa:yannubuntu/boot-repair && sudo apt-get update
  
  sudo apt-get install -y boot-repair && boot-repair
  
  

  
  



• After installation, boot-repair will automatically launch, if is doesn't, launch it via Dash.




• Make sure to select recommended repair to repair grub. Reboot and that's it.



• You should now have a Grub menu on boot, allowing you to choose from Ubuntu, memtest, and Windows 7

boot-repair did a really good job launched from a live-usb, by just applying the recommended option.

When you boot have you checked the boot order on your BIOS? If you're booting straight to the hard drive it will always miss the USB. You need to move the USB device boot option higher in the priority list.

Also check that your computer allows you to boot from USB in the USB port you are using.

In case your computer came with a pre-installed copy of Windows 8 and you removed it because to many people told you to hate it and you found that after you installed Ubuntu, that it isn't what you want, note this:

Your computer boots with UEFI and has a GPT partition table.

For installing Windows 7, either create and boot the Windows 7 installation media in UEFI-mode or set boot-mode and partition table to legacy.

To install Windows you need an empty partition that will be dedicated to Windows. If you do, then go ahead and install on this using the normal installation procedure. After installation, reboot into Windows and reboot a few times more to experience Windows ;-) (sorry, couldn't resist)

Like you said, this will break GRUB and your computer can only boot Windows after this. Do not despair, as there is a help section dedicated to restoring GRUB and being able to dual boot. Look here. After performing this, your computer will be able to boot Ubuntu as well as Windows.

Before you start, very very carefully note down the location (hard drive number, make & partition number within that) of the current Ubuntu install (which you don't obviously want to overwrite) and the empty partition in which you want to install windows. Also, during Windows installation, if it offers to format any partition other than the one you are installing Windows into, please do not accept.

The most save way is to install Windows 7 first and to reinstall Ubuntu after that. This will ensure that grub works.

You can also edit your partitions with a live cd, and then install Windows 7. When you have done this you boot into Ubuntu with a live cd and restore grub. This procedure however, is a bit more delicate than just installing them in the right order.

First, you have to boot with a live CD/USB stick and shrink your partition in order to create a second one. Windows 7 requires and creates a second partition which is called "system reserved". I don't know why, but it does. (So you will end up with three partitions or four if you have a swap partition.)

When your partition is ready, just boot with your Windows 7 DVD/USB stick and install Windows 7 on the new partition.

When Windows 7 has been installed, GRUB will break and you will only be able to boot Windows (automatically). Just boot with a live Ubuntu CD/USB stick and fix it (how it is mentioned in other comments).

Now another issue that some users may face:

I own an HP Mini 210 netbook which came with Windows 7. I erased everything and installed Ubuntu. Later on I decided to also reinstall Windows 7 and have a dual boot (needed Windows for a specific application from my university which wouldn't run through wine). At that time I had three partitions:

- Ubuntu

- Swap

- Backup/download storage

Now when I tried to install Windows 7 I faced a problem because Windows 7 needs to create a second primary partition (system reserved). I already had three partitions and therefore was unable to create two more. The workaround here is to create an extended partition in which you will include both swap and backup storage. Now I have:

- Ubuntu (primary)

- Extended

  - Swap

  - Backup/download storage

- Windows 7 (primary)

- Windows 7 system reserved (primary)

One good advice: When you make the separate partition for Windows 7, using gparted for example, format it to the NTFS right away. Do not use Windows installer to format the partition, because several times I had a situation, when the Windows installer corrupted the partition table after that and I had to restore the lost partition.
So, as the people recommended here: using gparted, create separate partition, format it to the ntfs, install windows there, then restore grub using livecd or liveflash. You can use the SystemResqueCD for liveflash.

I suggest you virtualize Windows 7 with something like VirtualBox (http://www.virtualbox.org)

This way you can run Windows and Linux at the same time, without the chance of destroying you Bootloader Grub.

I don't know why you want to run Windows 7, but if you don't need the full hardware capacity (Running the Latest 3D Games for example) of your computer for windows then virtualization can be a good solution.

Boot that USB

1. Turn on or restart the computer.
   


2. While the display is blank, press the F10 key to enter the BIOS settings menu.
   
   NOTE: The BIOS settings menu is accessible by pressing the F2 or the F6 key on some computers.
   


3. Select the Advanced tab using the right and left arrow keys.
   


4. Press Enter.
   


5. Use the up and down arrow keys to select Boot Order.
   


6. Follow the on-screen instructions to change the boot order so that USB is at the top
   


7. Press Esc.
   


8. Select the Exit tab using the right and left arrow keys.
   


9. Press Enter.
   


10. Press Enter.
    

Reboot...

Basically the same answer as @evgeny, but using chroot instead of a symbolic link.

1. Boot into a Ubuntu LiveCD


2. Open GParted and make sure you have an unused partition with at least 20GB free space. This will be your Windows 7 partition.


3. Install Windows 7 onto this partition.


4. Boot into a Ubuntu LiveCD


5. Make a note of the partition Ubuntu is installed on and any separate /boot partitions if applicable.


6. 
   

   Mount your Ubuntu installation

   
   
   

   sudo mkdir /mnt
   
   sudo mount /dev/<partition> /mnt

   
   


7. 
   

   Give this partition internet access

   
   
   

   sudo cp /etc/resolv.conf /mnt/etc/resolv.conf
   
   

   
   


8. 
   

   Only if you have a seperate /boot partition, otherwise skip this step

   
   
   

   sudo mkdir /mnt/boot
   
   sudo mount /dev/<boot_partition> /mnt/boot

   
   


9. 
   

   Set up then enter chroot

   
   
   

   sudo mount -o bind /proc /mnt/proc
   
   sudo mount -o bind /dev /mnt/dev
   
   sudo mount -o bind /dev/pts /mnt/dev/pts
   
   sudo mount -o bind /sys /mnt/sys
   
   sudo chroot /mnt
   
   

   
   


10. 
    

    Make sure grub-pc is the latest version

    
    
    

    apt-get update && apt-get install grub-pc
    
    

    
    


11. 
    

    Install GRUB

    
    
    

    grub-install /dev/sda
    
    

    
    


12. 
    

    Exit chroot

    
    
    

    exit
    
    

    
    


13. Reboot and check if it works

Installing windows after ubuntu is a pain but after you do this you'll need to put in your ubuntu live cd and install boot-repair and run that. Becouse installing win7 after ubuntu will break grub. But, boot-repair will remedy that!

Its a easy process to install dual OS. 1st create a NTFS partition from Ubuntu using Disk Utility. Then restart your system and continue with your windows installing. After finishing installing you can see that your GRUB is not working..... NOT A PROBLEM. I have two method to fix this GRUB problem.

Method 1

[Method 2] It may be bit difficult. Don't worry i have also a easy one. For that you have to login into windows. Download EasyBCD and run it. Go to Edit Boot Menu and set your desired boot option as default. Then save it and reboot your system. [Tested under windows].

Please let me know if you stack in something.