A good last resort (before pitching the pC) would be to download a little utility called "Insane Key Finder". This will identify your XP License key number.
Once you have that you should be able to use a friend's XP install disk and use YOUR License key number to install it. The activation process should bark if you haven't made any hardware changes.
The above steps will allow you to wipe the bloody thing clean and start over. Again...a last resort for sur...but I'd try a different browser like Firefox first.