You probably want to backup your data folders and files. One suggestion: install your HDD into a friend's computer and have them copy your data folders and files to their HDD, or to an external HDD.
If you have the XP disk from HP, do a repair install, then update XP at MS' Windows Update website.
If you don't have the HP restore disk, you should order it from HP, or you may have to use HP's restore feature to restore your computer to its original factory settings, and then reinstall any software you use that was not part of the original configuration.
Or, you could buy a new XP disk and try to perform a repair install, or, if that fails, perform a clean install.