Do you have system restore enabled? If so then try to restore your computer to the nearest 'restore point' . You can do this in safe mode too.
If that doesnt work,
Iam afraid there might be somekinda virus too ,loginto the safe mode, run adaware/anti-spyware/Anti Virus programs ,just to be on the safer side.
If that doesnt work,
Perform a 'repair' using the Microsoft Windows Xp Bootable CD.
IF that doesnt work,
Reinstall your Operating system.