When I turn on my PC the fans come on as normal but it wont even POST. It does this for anything between 5 and 50 tries, until it suddenly decides to work. I have tried everything: resetting CMOS, replacing it, unplugging every component and plugging it back in etc. I have heard it can be due to bad RAM, but I did a 15hr memtest with no errors, and even tried some different RAM, and it still does it. Anyone know what it could be?

What's the make and model of the computer, what version of Windows is installed, is this a new problem, was everything working well and this just started happening? Have you installed any new hardware or software recently? Sorry for all of the questions but we can't read minds.

OK sorry, the mobo is an MSI K8N-SLi Platinum, running XP MCE, it started happening a few days ago, everything was fine and suddenly it wouldn't turn on. I haven't installed anything new.