that little battery it is what helps keep the time correct when your computer is off.
and a few other little things but that would be one of the main symptoms

[Common Symptoms of the bios battery being week or exhausted If you boot the pc and you see a black screen and it says Bios Checksum error, thats a good clue, or you boot the machine and instead of starting the operating system you find it does not do so and when you go into the bios you find out that the date and time is not correct.
You then set the time and date to be correct save the settings to the bios and restart the machine but it still can not boot and the time and date you just set has been lost again and has gone back to 1st Jan 1980 or some such daft date. Look if you keep resetting the time and date in the bios and it keeps loosing it, then the bios battery probably needs changing.]
http://www.chephrenrepairs.com/bios_battery.html