Having run without problem for 2 years my system has started to exhibit the following symptoms around 2 or 3 times a day apparently quite randomly.
The screen goes black and a reset occurs starting with the BIOS splash screen - just as if the reset button were pressed.
It is a MSI motherboard P35/G33 Neo. Processor Intel Core 2 Duo E6750 @ 2.66GHz there are 4 DIMM slots each occupied by 1GB 800Mz DDR2 RAM. OS Windows XP Pro SP3 + all critical updates. Kaspersky IS 2010 reports no active threats.
If I run Memtest-86 v3.5 then a reset will occur within a few seconds of starting the first test - this is absolutely reproducible. I have tried rotating the modules (keeping matched pairs appropriately) this makes no difference. If I remove one of the modules (it does not seem to matter which) Memtest runs error free - have left it running all night but with unchanged memory configuration resets occur when running Windows.
I have reduced the memory to 2GB and still resets in normal use.
Have left computer running Windows in safe mode all night - no reset.
Any suggestions for resolving this problem would be much appreciated by an old man who is tearing his hair out.
Thank you both for taking the trouble to reply. The problem I have with the memory diagnosis, which was my first thought, is that it implies that all four modules have failed. This does seem somewhat improbable.