Thanks for the replies.
I got hold of my brother just now who, to be fair, has forgotton more about PCs than I will ever know.
He suggested trying the RAM one stick at a time in each slot.
It worked first time.
I have a bad RAM stick. Fortunately, the system will run on 2gb instead of the full 4.
Happy bunny.
Off to buy a new stick of RAM and r-kid a couple of beers.
