Comp repeatedly rebooting when logging in

This comp is pretty old, and is used by my daughter and granddaughter. I started into repeated reboots a couple of months ago, but I was just told about it. I cleaned out the machine and video card, both were VERY dusty, and I got the machine running while it was set up in my dining room where work on them. It was running fine, but when I returned it to my daughters room, it now is rebooting as soon as I log into her desktop. I realized that I had the side door off when testing in the other room, and taking the side door off has stopped the rebooting.
So it is a heat problem, for sure. Problem is, this machine is so old, HWMonitor cannot read temps from the processor. I have temps from the HD and Video card, but am not sure if the overheating is from processor or other components.
I have attached the .zip file with the info on the machine and the minidumps, but it looks like the recent reboots did not record a dump.
I would appreciate any help you can give me, if the dumps give any indication of the source of the overheating. Taking the machine apart is difficult for personal reasons, but I will remove the processor and replace the heat paste if needed. The heat sink is a ZALMAN CNPS 7500 Cu.
And before you all start yelling about replacing the computer, I just can't right now. I am working on an upgrade, but my granddaughter misses her games, and the upgrade is gonna take a while.
Thanks

Have you tried any other monitoring software, i am a fan of speccy (who also do a portable version )

If all else fails entering bios on boot up will give you cpu temperature although you wont be able to see temp under load