Okay, JB. From what you tell me I'm initially thinking a RAM woe. Try moving your own RAM card over to the next slot. If that fails try a known good RAM card that will suit your system.
If you have no joy this is hopefully a power supply problem. Borrow a friends good power supply and see how that goes.
If that fails. You might want to swear a whole lot 'cause we may be looking at your motherboard, but we will try other stuff first.
Are you familiar with your BIOs? If so you may need to enable your boot virus alarm just in case. I doubt it though.
Check your processor fan. It should be spinning without fluctuation (it does during boot up/wait till the machine is booted fully). It should be spinning like a mini jet engine then. If not, turn the machine off, unplug, unscrew the fan and clean it with the trusty little paint brush. Also clean the heat sink with one of those cool little mini air compressors. Please do not remove the heat sink unless you have to. You can blast it with the air compressor and vacuum up the fine dust with a normal vacuum cleaner... carefully.
Off topic but slightly relevent... If your processor is getting hot you will initially get "explorer is causing invalid page errors/faults in... ***.dll. The first will be kernel32.dll. The rest will be random .dll files. First, clean your heatsink!!! Your processor is overheating.
I get sidetracked so easily.
Get back to me if any of those don't work, JB.
Cheers
kiri