Okay, there are a few options, from what I see on the web. First, did this problem begin shortly after you installed some application? If yes, try to reinstall the application.
Secondly, if the above does not help, try to go to
Memtest86+ - Advanced Memory Diagnostic Tool and follow the instructions for downloading, and running a memory testing application. It looks like it needs complete access to the RAM (i.e. burn on a bootable cdrom or floppy (wow they still make those?)) and go from there.
Third: Have you checked your BIOS logs to see if there was a thermal shutdown? You raise the possibility that it was overheating. If so, your logs for BIOS should show something.
The chances are that the application recently installed corrupted, or it's Bad RAM. Of course, if the application is the OS, you will want to reinstall Vista.