Like these guys have said - the CMOS battery looks like a watch battery - silver disc somewhere on the motherboard. They do fail - but your computer should still boot - things like the current time will be lost, and possibly some other settings.
Here is what to look for:
http://www.google.com/images?q=cmos+...w=1490&bih=957
Finding a replacement could be tough on short notice. If you have any spare computer laying around you might be able to scavenge one from there.