by replacing the CMOS battery, or even by resetting the CMOS state.
The battery is the round silver one that's about the size of a 10p coin - usually it's a CR2025 type, and available from pretty well any supermarket's bettery stand. Just open the computer's case and take a look at the motherboard - the battery is very easy to spot. Just ease it out of its retaining clips and slip the new one in - make sure to get it the same way up as the old one.
Alternatively you could slip out the existing battery, wait for a couple of minutes and slip it back again. This action will drain the CMOS state and when you boot you should automatically load the default. Best to put in a new battery while the case is open though.
If that doesn't fix the problem come back here for a rethink.