If you ran a virus scan like I suggested, and nothing showed up...then I'd suggest a repair install. To perform a repair install of XP...in the BIOS (normally under "Advanced BIOS Features Setup Menu"), configure the computer to start from the CD-ROM drive (make the CD-ROM drive the first boot device) | Insert the XP CD while in the BIOS, save settings (F10) and reboot - (You can also boot with a 98/Me bootdisk with CD support and run Winnt.exe...located in the I386 folder on the CD).
When the "Press any key to boot from CD" message is displayed on your screen, press a key to start your computer from the XP CD | When you see the following message displayed on the Welcome to Setup screen, press ENTER:
To setup Windows XP now, press ENTER
At this point an option to press R to enter the Recovery Console is displayed...do not select this option | On the XP Licensing Agreement screen, press F8 to agree to the license agreement | Make sure that your current installation of XP is selected in the box, and then press the R key to repair XP | Follow the instructions on the screen to complete Setup.