Ok, you not only don't know how to use Linux, but it seems you also don't know how to use WindozeXP.... here's the most easy solution I can come up with:
You should have a boot floppy, or be able to boot with your WinXP CD (or you're on the ilegal side?? if so: keep using Linux, don't steal other's work!)
1) Insert either the floppy or the CD in the proper drive and reboot your PC
2) Follow the instructions provided by Micro$oft, and when it ask's for the partition where you want to install it (you should be seeing a partition manager of some kind), simply choose the Linux partition you created when installing Ubuntu and delete it, and after that simply choose the free space that this previous operation left.
3) If you created more than one partition when installing Ubunut, you may want to delete all the partitions to be able to use all your hard drive under XP...
Hope this helps...
PS: sorry for the harsh words at the begining of the message, but you also had harsh words for those who spent a lot of time working and gave away their work for free... and you had no reason to do so, since all this problem was only your fault.