Jeff has a good question. Are you testing the battery with a meter to see if it's really dead (under 2.6 volts would constitute a dead battery in my experience)? Or is something happening (like CMOS settings getting reset) and you're assuming the battery is dead and replacing it?
If you're certain the battery is really dead, do you, by any chance, leave the computer unplugged over-night (or turn off the switch on the power strip)?
Modern computers are meant to have power supplied to them all the time. The CMOS battery will discharge if you don't.
HOWEVER, this alone doesn't explain your problem because it should take a year or more to fully discharge a CMOS battery. I would suspect you have a power drain somewhere.
I'll give you one real world example I came across a few years ago. I had a client who would turn off the power via a powerstrip. This killed the power to the computer and monitor but NOT the printer. Due to a design flaw in this model mobo (a missing diode to be specific we later discovered), the printer would drain power from the CMOS battery via the parallel port in a matter of days. Very bizarre. No one else ever complained to the mobo manufacturer because, apparently, the conditions required were so unique (power killed to the computer but not the laser printer).
If you normally kill the incoming power to the computer when not in use DON'T. Just turn off the computer and see if that solves your problem.
Hope this helps...
