I was having all kinds of problems with my brand new Dell Studio Mini-tower. I think Vista is causing a lot of trouble. With all the security the program has, I think it causes the computer to crash a lot if you don't turn off the virus control when installing some programs. I was experiencing freeze ups using Explorer 7, that would lock up my whole computer and the only way to fix it was to shut it down manually. So if you're using that version, you might want to try this.
After wasting days looking for solutions online, trying repairs, scans, etc., I found out that the MAIN CULPRIT that was causing IE7 to lock up, was running IE7 in the "enabled protection" mode. As soon as I shut that off, none of the websites that were previously freezing up, froze anymore, and VOILA... No more lockups, so far!
I don't really think that should need to be on if you've got all the other security running, which should cover viruses, spyware, etc. I'm using Panda right now, by the way. Explorer 6 doesn't have that box and I've NEVER had an attack or virus on my other computer I've been using for 5 years!
Either double click on the area near the bottom of your screen that says "enabled protection on" or go into the Tools/Internet Options, uncheck the "enabled protection" box under the security level, and see what happens. Good luck!