Goliath. Meet David.

Yes, that's exactly what you think it is.  It's a battery.  Believe it or not it has a very important role to play in keeping your computer working.  That's the CMOS battery and its job is to power the internal real time clock and the BIOS memory.

Why is that important?  First of all the BIOS keeps the important information necessary to start your computer up.  Fortunately, the default configuration will almost always work which is what the BIOS will eternally route to without the CMOS battery.  Unfortunately your computer will always think it's fresh off the manufacturing belt and revert to the date and time hard coded in the CMOS.  Again, not really that much of a problem until you try to start it.

Yeah, it'll stop right there (at least it does on this computer).  But if you listen closely, it beeps once when it starts.  Turns out it's stuck there because there's an error message.  Good thing I know my keyboard shortcuts.  Alt + Tab will show me what's up.

Funny thing is if you actually do nothing for around 5 minutes it'll ignore this and just start up.

Now had this particular computer had a working laptop battery, you could set the time and restart it just fine.  As long as the system has an alternative power source it'll reserve some to keeping the time and date settings.

And why do we get this message?  Once Windows XP detects that your BIOS time is set before 1996 it triggers this error.

Fortunately for me, the battery on this particular laptop was pretty easy to get to.  I know a few models that require dismantling the entire blamed thing. Trick is now to find a replacement.  The battery itself is common (you probably have one in your wristwatch...if people are still wearing those); it's the attachments you've got to worry about.  I'm considering jimmying a standard CR2016 battery if I can't find one that's made for computers.  It it turns out to be too much hassle, we're looking at eBay.