Computer Maintenance 101

A few days ago our computer was experience some massive problems. It was experience really bad lock-ups. By a bad lock-up I mean the screen just froze. No mouse. Not CTRL-ALT-DEL. No Blue Screen. The clock even stopped. It would happen between 10 seconds and 5 minutes after the computer started.
At first I feared a virus or worm. Then I feared hard drive failure, or bad RAM. (Of course it has to be the hard drive...I haven't backed up for a few months...)
So I started in safe mode to rule out and bad device drivers. The computer didn't crash after an hour on, but the fan was going like crazy. We have one of those temperature sensitive fans that turns on when it starts to get hot.
Wondering whether or not it was hot, I rebooted into BIOS mode, and went to the section that told me the CPU temperature. Almost 70C (160F). I did a quick search on normal CPU temperatures, and I found the normal range was 30 to 50C.
I shut the computer off, and looked at the CPU fan. I could not see the cooling tower for all the dust.
I took the fan off, got out the vacuum, and vacuumed off the eighth inch carpet of dust from the CPU cooling tower (being careful not to touch any components inside the computer)
The computer has been fine ever since.