|First, let me say that I am by no means an expert, so take my advice at your own risk.|
Now, is this an older laptop or what? The only situation I can think of, outside of a virus because you say this is a clean format & install of vista, is a hardware issue. I have seen machines simply shut off like that because the cpu is overheating and it is thus protecting itself. I guess what I'm trying to say is it sounds like its a hardware issue. The only other thing I can see it being is a battery/power issue. Have you had the machine plugged in? Will the machine run without the battery in at all but still plugged into the wall?(don't know if this will work on your laptop)
Again, I'm no expert, just trying to help.