If your computer is more than two or three years old, a safe bet is that your CMOS battery is losing power.
(Time-out: CMOS refers to a chip that contains vital information about your computer's configuration and how it should boot up. The chip can hold onto this information only as long as its battery has power.)
The good news is that the battery is very similar to the lithium batteries that power digital watches.
Unplug and open your PC. Look for the battery, note what voltage it is and which side is up. Don't take it out yet.
Buy a replacement.
After you have discharged yourself of static electricity or have put on an anti-static wristband, replace the battery.
If you have been efficient - and even if you have not but own a fairly new-model computer - the next time you boot up, the CMOS chip will automatically regenerate the information it needs about your PC.
But there is a very small chance that it will not.
So, before you change the battery, boot the computer.
Watch the bottom of the screen for a message telling you which key to press to enter "Setup" - probably the "Del" or "Esc" or one of the F keys - and press the key before Windows starts.
Once the CMOS file is open, go through all its sections and write down all the settings.
After you have changed the battery, reboot, go into Setup again, and make the appropriate entries if your CMOS chip has not done so.
Of course, if you have a maintenance utility such as Norton Utilities, you can use it to back up and restore your CMOS settings.
One other thing: If your computer is more than five or six years old, you may find that the CMOS battery looks like a teeny flashlight battery and is soldered to the motherboard. Replacing it will mean a trip to the computer repair shop.