This works for removing Ubuntu OS from a dual boot system.
In general, you have to restore your MBR and reformat your Ubuntu partition
To restore your MBR:
1. Insert the Windows Setup CD into the CD or DVD tray on your computer.
2. Restart your computer.
3. Hit Enter to reboot from the Windows CD when prompted to do so.
4. Type “R” and hit Enter in Windows Setup to enter the Recovery
Console. The Recovery Console will appear and list the Windows OSs
on your machine.
5. Select the number corresponding to the Windows installation you
wish to boot into. For most people this will be a “1”. (In some
versions of the Recovery Console the installations will be
numbered. In others, you will have to deduce the installation
number from the order of priority in the list.)
6. Windows will prompt you for an Administrator password.
Try your Administrator password or, if your Administrator password does not
work, hit Enter.
7. At the Windows prompt, type “fixmbr”.
8. Type “exit” to restart the computer.