Well, it doesn't turn on. At all. It does the same with or without a hard drive in it.
The truth is, as crazy as it sounds, the MacBook died over the Summer. I took the hard drive out to back everything up and left it on the windowsill (not in the Sun). Three months or so later I still haven't got round to bringing it into a service centre (I've been using an iBook instead and just haven't had the time recently). I've finally decided to get off my ass and take it in, only to find that the hard drive has a dent in it (which wasn't there before). I think someone must have knocked it off the ledge and not told me about it.

I've got another hard drive I could put in but its not the same size.