I have a computer that crashes every 5 to 10 minutes. It just blanks out, and then starts up. When Windows starts up it says there was a serious error. Furthermore the error report window comes up about 10 times and every time I send it, it says the report is corrupted. It used to do it because it was too hot, but I don't think it mentioned anything about an error, and I put another fan in, which fixed that problem. I have speedfan installed, and it always says the temps are normal. Recently I installed Ubuntu on that computer, then installed Windows XP again, though I am not sure if the MBR was fixed (I assume it was, though).
Given that you have had overheating problems in the past it is possible that some component or components have been damaged irrevocably by the high temperature. You may have fixed the immediate visible cause of overheating, but overheating can cause much more subtle damage and this could be the slow manifestation of that damage.
It is entirely possible that this is simply something like a power supply fault and that is always a nice simple component to change, so I would start there.
It is also entirely possible that the components in your system are simply just getting old. Without details of your system or more detail of how it actually crashes it is difficult to point at any one component as being faulty.
At the very least we would need details such as CPU and motherboard, and any other relevant details you can think of.
Get an Ultimate Boot CD and test your computer for problems (HDD, RAM, CPU). If you get no crashes/errors during testing then it must be system problem but I would first try with hardware problems as Windows XP unless really badly maintained is pretty stable system and shouldn't crash on itself. Also I don't think it's MBR problem since you can boot the system up. Scan the hardware, it will get you the problem you're looking at.