Anyway, while vacuuming all the dust out of the computer, I noticed that the IDE cable wasn't plugged into the back of the hard drive properly. I suspect that may be what the problem was, but who can tell?
I ran Norton System Check again and it worked all right, so I defragged both HD partitions. Dad forwarded the suggestion that the shop may have left a layer of grease or something when installing the heat sink, which explain why the CPU overheats when I leave the cover on. Not entirely sure. I find it suspicous that it would reset specifically when reading/writing on the HD.