Edmilsom :
glamdringfh :
Did you perform a dual-boot installation of Ubuntu on the same drive you have Windows, or is that the only operating system currently installed on your hard drive?
Knowing this will provide a few more options for troubleshooting.
its my only OS . no duel boot
Well obviously you can't uninstall an OS from within itself.
Get yourself a liveCD of another Linux distro if you want that or download and burn an ISO for Windows and boot from those.
From there, you just need to format the drive (IT WILL ERASE YOUR DATA, BACKUP FIRST) and you're doing fine.