Hey everyone, first time poster, longtime noob at comps

.
Lately, my comp has had aggravating heating problems. Started at the beginning of this summer, ever since I added a second 120GB hard drive.
The problem is that it will suddenly shut off without notice. Once restarted, everything is fine, no disc errors or loss of data so I'm sure its not a virus. However, the system won't run for another 2-5 minutes without turning off again. I'm lucky I can type this out. Usually I have to leave it off for at least an hour to get 30 minutes running time, and leave it off overnight to get 5 hrs out of it. The usual cpu temp after restarting is 147 degrees farenheit (around 60 celsius), but it sometimes shuts off with low temps as well (around 46 celsius, once it even shut off at 25)
I've already opened the case for better airflow and got a giant fan blowing into it. This will usually make it last longer, but it doesn't fix the problem.
I've downloaded Speedfan and recorded idle temps , which is around 44 - 51 celsius ( with fan blowing into it, which is now somewhat necessary to keep it on, without the fan the temp will continue to rise until it just shuts off)
One of the fans on the power supply strangely stopped working, even after cleaning the dust out. I'm not sure what to do with it, or if I should replace it. (80mm)
Just a guess, but I think it may have to do with the power supply failing, since everything else seems to be working fine. I would prefer not to replace it if I can, I have no monies.
Please please please help, and thank you so pooping much for doing so. I can't watch my pron in peace :C
AMD Athlon XP 2400+ ( 2.00 ghz)
1gb ram
Two 120GB Hard Drives
and thats all I know off the top of my head