Unexpected shutdowns

> I've had an Ultimate instillation of the 64x RC1 since it was first
> released,
> and I've had a fairly decent experience with it, except for one thing. I
> can
> use vista for hours on end, and as long as it stays active there aren’t
> any
> problems. However, when I leave my system running idle for a few hours it
> will shutdown. When I reboot there is no indication that windows
> encountered
> an error, and the log files show nothing. This happens every time my
> computer is idle for more than about 3 hours. Is there an automatic
> shutdown
> feature that I'm unaware of, or is this a more serious problem?