Maybe you have a bad battery.

http://www.geek.com/sysup/upques132.htm (from google)

Does the system clock change or does this happen in windows only (I'm assuming windows)? For example, if you boot, I would check the BIOS first (especially if the clock resets are predictable) so that you can tell if it is windows changing your clock or if there is glitch somewhere else.

Also, Does the clock reset WHILE you're in windows, or on reboot?

...or you could just shell out $2 for a new battery and hope it works. Either way