After a couple of days of that, it got to the point where it would shut down, try to restart, the light would come on, and it immediately shut down again after a couple of seconds, and would keep on and on retrying, eventually coming back on after a dozen or so attempts, running Auto Repair and working. Tried a system restore, no good. Eventually got to the stage yesterday where it tried restarting and kept cutting off straight away, must've done it about 60 times. Managed to get it to start in some kind of HP test mode, where it ran a diagnosis and told me the Memory Quick Check failed, on memory module 2 DIMM2. Opened the case up, removed half a beagle's worth of fur which although not the cause of the problem, probably wasn't helping the situation much. There were 2 RAM cards (4GB each), couldn't tell which was number 2, so took one of them out, still had the same problem, put that one back in and took the other one out and it started perfectly first time. So, to get to the point, my question is, would the problem have been with the RAM card itself (in which case presumably I can just buy a new one and put it in there), or is it possible the problem is with the slot rather than the card (in which case I don't really know what I'm doing and will have to get someone to look at it)? And presumably in the mean time there's no problem with using the PC with just one RAM card instead of 2, other than the fact my PC will be slower than before?