You may find this helpful:
http://castlecops.com/postt35734.html
Here is the Reader's Digest version of the link. Someone explains the
same problem you are having. They are infested with spyware and
viruses. These programs have corrupted their system files as well.
After removing all of the spyware/viruses, the problem still persists.
The final fix was to do a repair installation of Windows XP.
To scan for spyware and viruses, download and use Ad-Aware from
www.lavasoftusa.com and run the online antivirus scan from
www.antivirus.com
To repair XP, boot off your Windows XP CD, choose to Install Windows, it
will find your existing installation and offer to repair it, press R to
repair and let Windows Setup fix things for you.