See if the System File Checker can help: from Start > Run run
SFC /scannow
to test the core Windows files. If this does not help you may have to
reinstall Windows as described here: "How to Perform an In-Place
Upgrade (Reinstallation) of Windows XP"
(http://support.microsoft.com/?kbid=315341).
Before you do so you can test if one of the applications running in
the background cause this. See "How to Perform a Clean Boot in Windows
XP" (http://support.microsoft.com/?kbid=310353).
