Thanks for the replies.

Regarding the temperature, I use a program called Speedfan to check the temp on my computer. It tells me that nothing on my computer is running hotter than 49 degrees celcius, so I think temperature is fine.

I took a look at the event viewer, and I noticed that under "System", I have a lot of errors with "ati2mtag". This leads me to believe that my video card drivers may be the problem (but not my video since, as I mentioned, I already tried different cards). I'm going to try switching to older drivers from a time before my system started freezing, and see if that works.

However, if anyone else can suggest something for me to try, feel free to post.