Interesting situation I had, where I needed to change the Date/time on my machine to diagnosis a time issue. Since changing the date/time on a Windows domain connected computer is a big no-no (if you don't mind the domain controller throwing security errors because of the domain security tokens are time sensitive).

Then to find out that changing the date/time on a Virtual PC host (not domain connected) does not work without a mod to the VMC file. By default the VPC guest syncs its time with the host PC every 10-15 sec. The best way is to update the VMC with the line below, under the <integration>/<microsoft> node:

<components>
    <host_time_sync>
     <enabled type="boolean">false</enabled>
    </host_time_sync>
 </components>

This will disable the time synchronization functionality of Virtual PC for this guest. You will need to stop the guest before you make the change and then restart the guest OS.

 Check out: http://www.nirsoft.net/blog/2008/12/running-operating-system-inside-virtual.html for other ways to change date/time in VPC host.