You're right...Windows does synchronize it's time with an external server.
I think the simplest method to temporarily prevent time synchronization is to block
the port that the computer communicates to the internet with to get the information.
This should be port 123 UDP. So, use a firewall setting to block outgoing communication
destined for this port.
Or, you can play with the w32tm utility to adjust your settings and disable time
synchronization as described here:
http://technet2.microsoft.com/windowsserver/en/library/b43a025f-cce2-4c82-b3ea-3b95d482db3a1033.mspx?mfr=true
