A few downloads you should attempt to help narrow down the problem.
http://www.download.com/CCleaner/3000-2144_4-10315544.html This will not fix the problem but it will speed up your system and make subsequent scans go faster.
http://www.mozilla.com/en-US/firefox/ It often rewrites over itself even if you already have it installed. Good to do just because...
http://www.download.com/Trend-Micro-HijackThis/3000-8022_4-10227353.html This is a good one. It's going to give you a lot of info. The best way is to run the program and copy/paste the results in in a forum geared towards it. There are a lot of people willing and able to help you with the results. I wouldn't suggest FR as a posting medium. Use your favorite search engine. Type in "hijackthis results" and find one you are comfortable with posting to.
I've had Zone Alarm get buggy with me on a cable modem in the past. Try to access the programs that it's blocking.
I've got more suggestions, these are mostly basic. Before you go to sleep for the night, this one takes awhile, run checkdisk again through your XP. Scroll down this link to "Manual steps to run Chkdsk from My Computer or Windows Explorer"
http://support.microsoft.com/kb/315265">http://support.microsoft.com/kb/315265
Try to get the hijackthis downloaded and running after cleaning first. You might be able to mitigate the problems with that utility.