As an alternative to changing the CMOS battery, if your system is always connected to the internet, i would suggest you to use software like BetaClock (which also offers a customizable tray clock display) or Neutron.
http://www.betaclock.com/
http://keir.net/neutron.html
Those programs can start with your Windows session and directly synchronize the system clock over the internet.
That's how I personally solved this problem.