If you've run the memory diag and the HD diagnostic with no cause for concern indicated...and there's no malware...and no overheating due to poor air circulation, you've pretty much eliminated my list of prime suspects.
You might try checking Event Viewer for possible clues, How To Use Event Viewer - http://www.bleepingcomputer.com/forums/t/40108/how-to-use-event-viewer/
Just for the heck of it, I would also take a peek at Device Manager, just to satisfy myself that I'm not missing any obvious indication of a hardware problem that it might present. I would not expect to find anything there, but I would look anyway.
I might also run the sfc /scannow command to try to eliminate the O/S from undue suspicion.
LEARN how to use SFC.EXE (system file checker) in this article! - http://www.updatexp.com/scannow-sfc.html
If all that yielded nothing satisfying and the BSODS continued, I would then try a repair install before pulling out the biggest, most guaranteed weapon...a clean install.
If that yielded no satisfaction...I'd have to start thinking about serious hardware replacements.
That's the path I take with these things...I've never had to resort to replacing a motherboard or a system or a CPU...but I have replaced hard drives, RAM, video cards, and power supplies. I'm not as gifted at diagnosing things as many others here, but I can usually determine what I should do by a process of elimination.