Usually that would only happen if the Cmos battery is weak. How old is your PC? And what Internet Synchronization page are you using? If anything, try "time.nist.gov" - I know that one works.

stonecold, go to Start -> Control Panel -> Date and Time (under "Clock, Language, and Region" in Home view). Select the "Internet Time" tab. It should say, "This computer is set to automatically synchronize with <name of ntp server>". If it doesn't, click the "Change Settings" button and check the box, "Synchronize with an Internet time server". This will cause the time to be synchronized over the Internet on a regular basis.

Does your system lose time when it is powered off or powered on?