Chances are you've only destroyed the Windows XP volume or partition boot sector, I suppose this could make it so you can't boot depending on your setup. Here's a page that describes how to restore it: http://pcsupport.about.com/od/fixthe...bootsector.htm
Some more information about your setup would help us give you further instructions
Which bootloader do you use? LILO, GRUB, or the Windows one?
When you say you can't boot your system, what actually happens?