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?
The hard drive or the motherboard could be going bad. Disconnect the hard drive to see if you can get into the Bios normally or if it still shuts down or misbehaves. That at least will tell you what your next move should be.