Mine is running “funny”. It’ll stop every couple days (or slow down) and lose time, about half an hour or so.
Is my CMOS battery going out? The computer’s about 5 years old (Dell 1501)
I don’t want to open it up if I don’t have to…
Do you turn your computer off regularly? If so, and if it seem to lose time when it is off, I’d say that chances are very high that it’s the CMOS battery.
What Knead said - especially if you turn the computer off with a power strip, wall switch or something other than doing a system shutdown or using the front panel off switch. When you just do a shutdown but leave the computer plugged into a live outlet, some power is still going to the motherboard and it won’t have to rely on the cmos battery. Note: that may or may not resolve the issue though.
If doing such a “soft” shutdown isn’t an option, there are many freeware utilities that will go out to the internet to get the correct time and automatically update your system clock at whatever interval you choose, including at startup. Go to download.com, for license type select ‘free’ and do a search on ‘set clock’ or something similar.
Nope. I leave it on always. Actually, it just lost 3 min. since I posted. 
A slow computer clock is a sure sign of a CMOS battery going bad. Five years is a bit short, but not drastically so. Replace it and the problem should be fixed.
@Jake: RC is right. I know you’re reluctant to pop the hood and go groping around inside, but it’s really not that bad. The scariest part will be what they charge at Radio Shack for CR2032 and CR2025 button batteries.
But try this first. See if you can find what motherboard you have and then go to the manufacturer’s web site and download the manual. Look up how to reset the CMOS - since this almost always involves popping out the button battery. This will give you an idea of how things are laid out and what’s involved.