Re-install XP on the original partition. This will wipe out the Linux boot loader - either lilo or grub - so you will then have to re-install, or at least repair, the Linux installation.
Don't let either installation re-partition the disks, or erase anything, as they seem to like to do - unless you have screwed up on a larger scale than is evident from your post, this won't be necessary.
Both the distributions I've used - SuSE and Mandrake (now Novell and Mandriva) automatically check for the presence of a Windows boot loader, and - if one were found - both the Linux booat loaders I've worked with (lilo and grub) would then offer you a windows option at log-on.
What sort of crap installer are you using that managed to over-write the XP boot loader?
For more detail try
------------ Bill Sloman, Nijmegen