The full solution is detailed on this post on Prasy's Blog. Basically, there are two solutions possible for fixing the problem in Windows: either run the Boot Camp helper that's included on retail Mac OS X Snow Leopard discs, or edit the registry key that determines how Windows sets the time. The registry method is preferred, but it's definitely more complicated than the Boot Camp solution.
The blog also provides a full explanation as to why the clocks between Windows and Mac OS X go out of sync:
"OSX uses UTC (aka GMT +0) to store its time. When you set your time zone for instance to +8 GMT (Singapore), it stores the time with reference to UTC. When you boot to Windows, the time that you’re actually seeing is UTC. But if Windows sets the time, it makes OSX all cranky. So the only way (well one of the ways) to fix it is in Windows. Another thing to note is that your BIOS clock will be in UTC and not in your local time (but don't worry about it, both of the OSes will understand that your BIOS clock is UTC)"SOURCE: Fixing Time Sync issue with OSX/Windows & Linux