Thanks I've just tried removing my tv tuner card, 56k modem, graphics card, hard drives (I have 2 x Seagate 200Gb).
I then removed my mobo battery and removed and replaced the cmos jumper, switched the monitor to the onboard graphics card and managed to get the system to boot. I then replaced the system HD and managed to get to windows (xp). I have yet to try my second HD (I'm scared!).
I'm not sure what has caused/cured the problem, or how to go on from here?
Also I have a micro ATX case, which with my PCI Express card, modem, 2Gb Ram, 2 x HDs, TV tuner, 450W PSU and 3.4Gb P4 processor in there, could it be a little too much and although I have 2 case fans....? Could this be the cause?
Thanks for all the help so far.