Sounds like your CMOS battery might be dead. The CMOS battery is what keeps power to your board to store those settings even when being unplugged. If you go check your BIOS settings do you find that your settings are reset again? Are you unplugging your system at night? I only ask because some people do kill all power to their systems when not in use to prevent any possible surges.
If you are unplugging your system at night or for some reason you lose power to your system then the power will drain out of the motherboard and your BIOS settings and also Windows Date/Time will be reset. Symptoms definitely point to CMOS battery with the only oddity being that typically desktops aren't unplugged. If it ends up being the CMOS battery you can just pop those out and take it down to a Batteries Plus or similar location and get a new one pretty cheap.