The CPU fan is probly the problem. The fan blowing air out the back is a seperate fan.

I've seen more than one CPU get cooked due to no air on it.

I've got an AMD 1Ghz and the life of it without fan and heatsink is something like less than a second.

You aren't going to hurt anything by opening the case up and looking at it. You'll be surprised how much cat hair and dust will need cleaned out of there. You can also get those cans of compressed air to blow out the heatsinks and stuff too.

Just remember heat is what kills electronic components!!!