Hi, Soz.
WinXP has a feature called WFP (Windows File Protection) to guard against deletion of critical operating system files. In most cases, it won't let you delete or change one of those files. Just in case, it keeps a "known good" copy on the hard drive, usually in the \Windows\system32\dllcache folder.
You should run the System File Checker. At the Run prompt (or in a "DOS" window), type: sfc /scannow
SFC will compare each WinXP file to the copy in the cache and replace any missing or damaged ones. Have your WinXP CD-ROM handy; SFC probably will need to see that, unless your computer came with WinXP pre-installed, in which case the original files are probably in a hidden partition on your HD.
If this doesn't work, you may need to do an in-place upgrade, as described in this Microsoft Knowledge Base article: How to perform an in-place upgrade (reinstallation) of Windows XP
formatting link
This will reinstall WinXP itself, but preserve your installed applications and data. You will need to visit Windows Update as soon as you get your firewall and antivirus back in place and get back online, to be sure you have all the latest updates.
RC