I'd also go along with HDD going to sleep. I'm not totally convinced the MS have got these power-saving routines down pat. My computer sometimes crashed totally when it was trying to wake the HDD up. I switched off this option and, scorchio! no more crashes. It has to be said, though, that I was running the SETI@home screensaver, so maybe the buffer got filled up.... Hmmm hadn't thought of that before...
TTFN