I am trying to get a friends pc up and running again.  The issue he is having is random reboots a few minutes after booting into windows.  Its also reporting memory issues.  My friend has tried different memory in the pc and that has given the same errors.

His first thoughts were with the problem lying with the motherboard, however, in retrospect the memory controller is located on the CPU.  How likely is it that the problem lies with the CPU over the motherboard or vice versa?  I havent had a 939/AM2 system so I dont have any experience in troubleshooting this scenario...

He apparently keeps the pc in a cupboard so its out of sight, it also means it runs very hot.  60C on the CPU...  The cooling is something that will be looked at if I can get this PC up and running again.

 I would be more inclined to think it was the mobo rather than the processor tbh (unless it is a straight heat problem)

The PSU could be a issue check to see if it's overheating and have a look at what the lines are putting out in Bios.

I know you have already swapped out the Ram but I would run http://www.memtest.org/#downiso which can also detect any mobo or CPU problems, and would also rule out software and drive issues.

Thermal cutout on the majority of the Athlon 64 CPUs is 70*C if my memory serves.  If its idling at 60*C then under load its likely hitting its max heat.  Clean out the CPU fan and use some better paste perhaps?