How fast are we talking? Minutes or hours? If it is hours, make sure your time zone is set correctly. If it is minutes, you probably have a software issue causing it (<cough> Norton <cough>). It is easy to verify. Set the right time in the bios, reboot, go into bios and let it sit there. If it keeps time, then you have a software issue. There are also lots of previous posts on similar issues, might want to give the search a try and see what you can come up with.
I would like to add that it is not the cmos battery. It is never the cmos battery. Unless your time is going completely crazy, or you are losing bios settings, it is not the battery. (not that you said it was, just in case)