Spooky Vista

Last night my main laptop, which runs on Vista Home Premium, did a funny thing which it seems to every month or two. Basically, after switching my laptop off and going to bed, I am awoken by the laptop switching on again about an hour or so later. It's a complete pain in the ass as I then need to get up, login and then hibernate it again (I normally hibernate so I don't have to load all programs again).

The laptop is completely switched off and isn't updating or anything like that, so I'm completely baffled why this is doing this. Why is it just randomly switching itself back on. Anyone got any ideas?