Likely hardware but thought I'd throw out a chance.
I don't usually have the battery in and charging all the time whilst using the lappy on AC but I was about to take it out so had it connected. The nipper came past at a rate of knots and yanked the power cord out. I would have expected the lappy to have stayed on as it had been charging the battery for quite a while, but it powered down instantly. Reboot was fine (Vista Home Premium) but I then noticed it wasn't (and probably hadn't been) charging the battery. Whilst plugged into AC I took out and put back the battery and that set it off charging again. Or at least the tray icon said as much. I went into control panel to look at the power options and it shut down instantly. This continues to happen now. Any time I try to do anything of any substance (for example, I tried to copy a folder of photos onto a pen drive) it shuts down. I managed to get data off the drive by using safe mode, but I'm not confident it wasn't anything more than fluke.
Fans working, memory seated well. Unit is just out of warranty.
Anyone have any theories? My guess is the power socket joints have suffered from the yank and possibly lifted someway from the board giving an intermittent supply (battery dead/not charging so unit powers down).
Probable insurance replacement at the end of the day.