A friendly suggestion: make sure every scrap of data is backed up.
go to vista, if hp start menu make recovery discs. or similar for different brands.

be warned: 90% of the time in dual boot vista will ask to run disk check on startup and become VERY unstable.

use bootus and install bootmgr to make sure you CAN boot. Some cases dont let you go to repair pc or safe mode because it cant find ANY operating system.

Try slipstreaming your XP. This sometimes helps for some weird reason.