Hi Thanks for your replies The PC is at my Mother in Laws. I am unable to get there at the moment. (being a full moon and all that). But will try over the next couple of days and let you know how I got on.
Do you get any beeps when the PC fails to boot? It could be faulty RAM as when you restart, the RAM doesn't lose power. When you power the whole machine off the power is cut so it's probably having a problem initialising the RAM. If you get one long beep, that's likely to be a RAM problem and you should try another stick of RAM to see if that helps.