I have attempted to remove Ubuntu from a dual boot system (Windows XP, SP3) by deleting partitions, using both the Ubuntu install CD and GParted. Neither worked for me; instead, all I did was manage to install a new version of Ubuntu each time. Now, I have 3 versions of Ubuntu and one of XP. Trouble is, I cannot boot up XP. Each time I select it from the boot list, a message appears briefly saying chkdsk needs to be run, but instead of running chkdks, my computer restarts. I can boot up Ubuntu, so I have a way of getting online, if that is needed to find a solution.
This will erase all data on your computer
If for some reason you follow each of the directions above and your computer still boots into ubuntu or windows then you likely have more than one hard drive.
Alternative method (faster)