My Win 7 Professional desktop keeps on shutting down. I thought it was heat-related so I cracked open the case. There's no heat. I blew it out with compressed air. It began to forget the proper boot drive, the date and the time. I replaced the CMOS battery. It ran great for about two hours. It shut down and now attempts to reboot for about a second, about every five seconds or so, but the fans only spin for about a second or so. Additionally, the C: drive only has about 23 gigs of free space. What could be going on?