Hi, Marc.
See this KB article:
How to Perform an In-Place Upgrade (Reinstallation) of Windows XP
http://support.microsoft.com/default.aspx?scid=KB;en-us;q315341
Note that this is NOT intended as a time-saver on reinstalling WinXP itself;
it will take about as long as a clean install. But, it should preserve your
installed applications and data - and most of your tweaks. As soon as you
get your firewall and antivirus working again, get online and go to Windows
Update to get SP1 (unless it is included in your WinXP CD-ROM) and all the
later fixes that protect you from the latest viruses and worms. The
reinstall will take an hour or two, and downloading and installing the
updates will take an hour or two (even with broadband), so you'd better plan
on half a day for the whole project.
If the problem you are having is in the WinXP files, you might want to try
the System File Checker first. At the Run prompt, type: sfc /scannow
SFC will compare each WinXP file with the "known good" copy held in your
on-disk cache and replace any missing or damaged ones. It takes a while,
but not as long as a reinstall. Have your WinXP CD-ROM handy; SFC probably
will need to see that.
RC