Sounds nasty, and it sounds like you have done plenty to try and rectify the problem. I'd have a look on the Microsoft knowledge base to see if there is anything there.
The only other thing I would do is...REFORMAT. Make sure you have backed up all your important stuff before though because you will wipe your hard drive clean and nothing will be left on it. Maybe take the chance to update the BIOS too if you can find a newer version to what you have. I find XP very temperamental when it gets a problem it really does screw up. If you have Roxio installed this maybe causing a few issues and I would try uninstalling Internet Security too.
Need more help then please email me.