I'm computer savy enough to deal with the usual spyware, VIRUS, run of the mill problems that normally happen on a computer but this one has me COMPLETELY STUMPED!

I am having a problem with a number of file types on my computer. I am USING Windows XP and Service Pack 1. (I wasn't too thrilled with service pack 2 and because of the security involved, alot of things on my network failed to work properly, so I uninstalled it)

Specifically, MP3 files.
I am able to open mp3 files as long as everytime I want to open one I have to select a program from a list of programs. Windows tells me it "Windows cannot open this file" and lets me select from a list of programs. I MAKE SURE I check the always USE this program to open this type of file and then the file opens. Problem is, no matter what I do, windows will not open that file automatically, even if I make sure Windows Media Player is my default player. I also went to FOLDER options and noticed the file type MP3 does not exist. So I added it. Even after adding it, it does not allow me to change the program associated with it and the next time I go into file types, the MP3 type is again not on the list. I first noticed this with just the mp3 files. Now there are about 3 types of files total that will not allow me to associate a program with it. The icons all go to the default icon that windows gives if you don't have a program on your computer for that type of file. I have software for every kind of file that i am having these problems with. I also did a registry cleanup with Registry Mechanic and that didn't help at all. Please help me as I am extremely confused as to what is going on here.

(I don't know if this is worth mentioning or even a possible concern, but I am hooked up on a remote network in Buffalo that connects to my corporate office in Los Angeles, through a VPN Tunnel. I am running through a Sonicwall TZ170 router) I mention this cause I'm not sure if the connection has anything to do with the settings on my computer.  First things first, SP2 is not an option, it is a necessity. I have never yet seen it fail to install unless the system was already compromised, except in the CASE of some realtime AV monitors being in place.
I've not seen many problems with it that cannot be easily resolved and believe me, I've seen a lot of machines since it's release!

What anti virus/anti malware measures have you taken?Did you reinstall Windows XP SP1 after you uninstalled SP2, or did you just uninstall SP2?

Your local area network connection problems with SP2 may have been caused by the Windows XP built in firewall that is installed with that service pack.  Disabling the Windows firewall or changing the firewall settings may have fixed the problem.

Doc