You may wish to try the System File Checker
http://support.microsoft.com/default.aspx?scid=kb;en-us;310747&Product=winxp
(In brief: from a command line, run sfc /scannow. You'll need to have the XP
CD on hand if any files are to be replaced.)
If you need more drastic repairs,
http://support.microsoft.com/default.aspx?scid=kb;en-us;315341
"How to Perform an In-Place Upgrade (Reinstallation) of Windows XP"
The knowledge base article is somewhat confusing (to me), but, in brief,
Change your BIOS settings to boot from the CD drive. Boot from the XP CD.
Choose to install, rather than going to the repair console. Under install,
choose to repair the exiting installation rather than doing a new one.
You will have to activate XP again. You'll also lose everything installed
through Windows Update.
Most settings, files, and installed programs will be preserved, so it's less
work than a reformat/re-install.
You may ultimately need to do the reformat, though, if your problem is a
corrupt Registry rather than bad files.
Good luck.
Bob Knowlden
Address altered to avoid spam. Replace nkbob with bobkn.