Hoorah...Problem solved.
Thanks Guys for all your help. I did actually have a copy of Memtest but hadn't run it as to me it didn't seem like a memory problem.
1X512mb stick is completely borked and fails every test at every address, the other is ok. The good news is they are only a couple of months old. Bought them as a matched pair so they will both be going back tomorrow.
Bit annoying though as a thought I was buying top notch quality stuff (£180 quids worth of Crucial Ballistix DDR400).
Thanks again.
