If you haven't updated your OS to SP2, or if you have and the disk HAS SP2 slipstreamed, simply put the XP CD into a drive, close down the window it pops up, and open the Run window.
Type sfc /scannow and let it run. Takes a while.
Just like in Win9x, it searches for b0rked files and will give you the option of replacing them.
Sounds like a virus got into your explorer.exe, though it could be straddling a bad sector.
Another way to fix this is if you have another PC handy with the same version of XP running.
Just copy the explorer.exe to a USB drive, a CD-RW, or whatever, then boot the problem machine into Safe Mode and rename the Explorer file, then copy the new one to its old location.
If that doesn't help, you can always rename the .old one .exe again and let it replace the one you copied over...