This computer I bought off ebay is an interesting beast.
Firstly, it would over heat and turn off: Fixed by replacing the crap fan on the CPU
Second, the BIOS went [MD5sum errors and conflicts with other hardware]: Fixed by altering the BIOS and leaving the comp turned on at the mains, but not actually on [naughty I know]
Third, Windows will now for any unknown reason, reboot. Then complain the BIOS is ballsed [WTF is Windows doing looking at my BIOS??]: Sorta Fixed, run Linux all the time [average Windows uptime: 20 mins, average Linux uptime: 12+ *hours*]
Fourth, only started last night. It turns itself on. Granted I leave it connected to the power to keep the BIOS alive, but switching itself on? I can only think its a BIOS power option [which I've found and turned everything off] or the power switch is dodgy and shorting out turning it on. Or I could have virus somewhere [which would annoy me, I don't get virii any more, far too careful]
If it is a virus, its a complete waste. The computer boots to the OS chooser [LILO], and sits there. Doesn't do anything else cept eat power and display a pretty red box with Arch, Lapwing and WinXP to choose from. Ah well.
