If you leave your computer on all day or for a long period of time, you may notice that the Windows clock on the Taskbar may lose time, perhaps two minutes per day, or perhaps an hour or more. This can occur for various reason, as explained in the following Microsoft Knowledge Base article:
http://support.microsoft.com/default.aspx?scid=kb;en-us;189706
Windows Clock on Taskbar and in Date/Time Tool Loses Time
The time of day is maintained by a battery charging the CMOS. One reason for a time loss could be the running down of the battery. Since the CMOS does maintain the correct time (if you restart the computer, the Windows clock is updated), you can determine if the problem is a weak battery in this way: click on the Start button, select Programs from the Start Menu, and click the entry for MS-DOS Prompt. At the DOS prompt, type the command TIME and press the Enter key. If the time reported by the TIME command differs from that on the Windows clock on the Taskbar, you should replace the computer's battery. Type EXIT and press the Enter key to exit the MS-DOS Prompt session and return to Windows.
There are other reasons why the Windows clock may lose time. For example, if you have Advanced Power Management settings enabled in your computer's BIOS, rather than in Windows, this may interfere with the Windows time. It can also happen with some third-party antivirus programs, utilities, and screen savers.
In order to let Windows control Advanced Power Management, you click on the Start button, point to Settings on the Start Menu, and click Control Panel. Double click the Power Management icon, select your desired settings and click OK.
Hope this helps