Ok, thanks for the help on this thread so far lads. But ive still got a problem.

Ive been able change my memory settings to what they should be. Ran a load of memtests and all confirm the memory is working perfectly.
My Quad Core is running between 45 and 60 degrees (depending on load) which is what id expect as its only using a standard Retail cooler in a Thermal Take LAN-Boy case (the only heat sink that ive found fits atm).

Built both a new XP OS and a Vista OS on seperate drives, so i can fiddle around with settings. I disable one drive to work on the one OS so there is no crossover.
Ive used drivers from both the CD provided and the Drivers online and all appears to be working perfectly fine. Full system spec is as follows.

Thermaltake LAN-Boy
Intel Core2Quad Q6600 - stock voltage
GA-G33M-DS2R Mobo
G.Skill 2Gb ram - 2.2v - 4-4-4-12
500Gb Seagate, 320Gb Maxtor
Pioneer DVD-RW 212 & Samsung DVD-RW SH-S202JBEBN
BFG 8800 GTS 320mb - 162.18 WHQL XP Driver
Thermaltake 750W Toughpower PSU

Now here is the problem.

The PC will reboot if play any games for a prolong period of time. eg a game of Supcom. It Blue screens but its so fast i cant see the error. When windows comes back up it says its suffered from a 'Stop Error' caused by a device driver. But doesnt tell me which driver.
Media Player and VLC player will fail if playing any HD avi at full screen (24" Dell) but will play as long as i dont full screen the program.
3DMark 2006 will fail towards the end of all the tests it doesn. But if i run the bog standard tests they run to completetion.
Orthos or Prime 95 will run for several hours with no errors so far. Ive not tested any longer than 3 hours using these.

So all my tests so far are showing that the CPU and Memory appears fine. But some thing else is causing it to reboot. But windows cant tell me what. Im thinking Graphics card or GPU driver atm. Can any one offer me some suggestions.

Note: Im not sure if i should create a seperate post for this or not. So if the Admin feels that this post needs moving please do so.

Many thanks.