Try running a chkdsk? It could be that you corrupted or deleted explorer.
If this doesn't work, you will probably need to replace the file.
Replacing it shouldn't be that hard.... What I would try is: Go to another computer with win XP, search for explorer.exe, write down the filepath to it. Copy it over to a flash drive or something and then copy it from the drive to the same location. Obviously, however, you can't access My Computer or anything, since Explorer is the file in need, but there are ways around this. Try some sort of bootable disk. Windows Recovery Console is perfectly viable, its just text driven. You can also try any sort of Linux off a Live CD, or the Ultimate Boot CD (also a "Live CD").
www.ubcd4win.com . This way is the easiest, I think, but you have to create the CD first. Its very easy, and the site explains it well.
Are you framiliar with the Recovery Console? (Boot from your XP CD)
Do you have a Live CD or UBCD?
Post back and I'll help you through it if you need it, but you should be able to find tutorials, etc, on google for all of these.
Whatever method you pick, launch the CD/Recovey Console, navigate to where Explorer.exe should be, and copy the good version from the drive/disk to the location, deleting the bad version if its there.
Good luck! Post back with questions!
All the best,
--The Comodore