Pop back in your Install CD (WINXP) and boot it. Select recovery console when it asks you what you wanna do (I think the choices are "Install Windows", "Repair Install", and "Recovery Console". This basically gets you into a cheap Dos prompt with some fancy commands.
Does XP try to boot? (EG, read the bootpartition?) If not, there's a command to rebuild your boot partition. If the MBR is screwed, you can rebuild it. You can also fix the files that make XP boot (the dreaded NTLDR error X( )
If your windows system is totally screw, reinstall it to another folder on the same drive. It'll say your private files may get over written. Simply choose another user name for yourself (maybe Remote

) and continue the install. When everything is up and running, all your old junk will still be in the old folders of your user name. Copy everything over and delete the old crap. You may have to pull some permission strings, but it's do-able.
NOTE: Don't choose the same user name that your private files are on, cause I -think- it'll delete them. I haven't ever tried it, cause I value my files
way too much
NOTE2: The install give you the option to reformat your system...please tell me you are smart enough to know not to...
(^^Those are for stupid people that may read this...I give you more credit than that)
If your filesystem isn't NTFS, you may wanna backup your files, and do a fresh install with NTFS. Unless you have a specific reason of using FAT, switch. NTFS is almost bullet-proof (I haven't killed it yet, and I'm rough on systems) and will treat you alot better.
Adam