There's about a dozen ways to go about this, the least intrusive one (though granted not very likely to solve this) would be to start with a simple startup repair, you'll need your windows disk for this.
I am hoping you didn't install linux's grub during any of this, I'm also hoping you were careful enough not to actually overwrite win 7 when installing xp. Startup Repair
now if that fails, and it very well might.
This is probably the most hassle free fix for you. Repair Install
It will reset windows itself to default settings but your programs and files will remain as they were.
There are some other less drastic methods but I don't have a whole lot to go on here and a repair install will fix anything you've managed to mess up with the xp boot data.