XP Pro or Home?
Computers are strange and wonderful, it wouldn't hurt to try reinstalling the hardware physically.
If you are really convinced that the problem is tied to the current user registry hive, then create a new user, install the hardware, bounce(i meant restart) the box, logon as the new user and see if the problem persist. For kicks and giggles, disable your security softwares during this test. You seem to have some good security software, but it could be a case of combinations.
If you aren't logging on as the local administrator (you better not be

), I would try using that account as well.
If you think you may have botched up win sys files, you can extract original files from the CD with something like 7-zip. If you are running a service pack, then you will need to extract the .cab files from those .msi installers as well. Then you can use something like windiff to compare the system directories. There will be a multiplicity of disparities depending on how and what you've installed. With some practice, you'll be able to sort through the normally messed up and new and improved messed up .dll's and executables.
It is possible to remove all sound capabilities from an XP machine, but since things like the media player are embedded in internet exploder, it's similar to slamming your dink in a door repeatedly.
Normally, I wouldn't recommend this, but you could reinstall the operating system right on top of itself. Boot from your CD, DO NOT format, and install to the same win directory as your existing installation. BTW, this is not the repair option. You will have to re-patch the OS, and possibly have to reinstall some software. Typically, I would mount the offending installation in other machine and copy the entire drive bit by bit before attempting this method (including hidden data). And, it's worth mentioning, a complete wipe of the drive and reinstall of the OS is just a little more effort and much cleaner.
Educational and a great computer forensic tool box, check out Helix. It's been a good while since I looked, but a Bart's PE used to have some drive recovery tools( like disk to disk copy).
Do you have a second computer?
Anything mission critical on the machine? Great rule - back it up before you jack it up. External hard drives are getting cheaper. Just remember, hard drives have a 100% fail rate. It's not a question of if. Secret squirrel stuff, good time to make multiple copies and encrypt.
Let me start pilfering through my stuff. Trade isn't necessary. Call it a late Christmas present.
While I'm looking, anything else on your wish-list? I was thinking of selling in bulk, but if it helps out someone, that's worth more to me than selling it to someone who probably doesn't need it. My wife will probably thank you since the computer graveyard/ lab/ office/ workbench is about hip deep in parts and pieces.
Tell you how bad it is, found 2 $500 hard drives in there last weekend, wonder what I did with that SCSI controller? I think someone gave them to me as payment since I hardly ever charge folks.
Edit: Since I didn't get so much as a sniff on the Ubuntu name drop, I thought I should mention that I am a MCSE (normally I don't tell people, and yes it's on current platforms and not from 1995) and shortly, former M$ partner. I work in a M$ shop. Your computer shipped from the factory with a Celeron and Win ME (Mistake Edition). These two should have convinced you

Oh, you seem like a nice guy, but if you buy an emachine again, someone should have an intervention. But I have faith that you too will see the light of open source. Sorry for the excitement, but I'm a recent convert and have been kicking myself for not making the switch sooner.