Do you have windows vista boot disk ? The above process works flawlessly with that I've not used it for xp ? What I said was boot into command prompt and type the above commands. Whats going on is ubuntu has Grub loaded into your Master Boot record, which is different from windows thats why it wont boot.

Edit:
Google around, you will find lots of ways to fix the boot record issues after ubuntu.
hope this helps.
http://bobmorris.wordpress.com/2008/02/09/how-to-uninstall-ubuntu-on-dual-boot-windows-xp-using-windows-only/