I got a nasty virus last year that wiped it clean. I have formatted the hd several times and reinstalled WinXP. Everytime I leave the computer off for more than a few hours, it won't boot up properly. It will try to boot up from the floppy drive and I can't get past that. If I hit ESC while booting up and change it to something else, it still doesn't work.
Sounds like cmos battery could be dead then. Should just be a battery that is the same used in watches (flat and similiar in size to a dime or nickel). If you can go into the BIOS and have it redetect the drives and it works fine till you power it off then thats most likely the problem. Check to get the # off the one in there and head to target/wal mart and find a replacement battery.
To get into the bios will vary by pc but it should splash some screen and say hit F2 or something to enter setup.
2007 Taurus SEL - daily driver
1974 El Camino SS - 400sb
You get into the bios different ways on different computers. normally its escape or an F key (F1,F4,etc.). I'd think you could look at the time when it does boot up to see if your CMOS battery is going.