All games crash on my system!

Hello Major Geeks Community.

My computer setup is as listed in my sig area, and virtually any game I try to play crashes with the standard Windows application error that it has to close and Windows is sorry etc.

The games I have tried with the system are TES IV: Oblivion, Quake 4, HL2, FEAR/FEAR Combat, Hitman Blood Money, BF2/SF/2142, Age of Empires, (any) GTA Vice City, Postal 2, Doom 3, and Prey.

The only exceptions to that list are that Oblivion can run fine for a long spell sometimes and the same with FEAR Combat. (eg, 2 hours online last night was fine)

Just to make matters worse, I recently bought WoW, to which I got tons of Installation errors. I checked the Blizzard FAQ's and they said it may be down to my RAM being faulty. They recommended to download and install Prime95, a free RAM checker.

I ran it for 40 minutes until it crashed; the test type was a torture test which utlised over 75% of my available RAM. (about 1.5GB) I read around some more and tried using another RAM utility called memtest86. This ran for over two and half hours and in that time completed four full tests - all were ok.

My dilemma is as follows:

What could be wrong with my system? RAM? Graphics? Or even motherboard?

Thanks in advance for your suggestions.

- krypton_ls