When you have Virtual Machine Additions installed on Virtual PC we will automatically synchronize the virtual machines time with that of the host operating system.  For most people this is a "good thing" - but if this is something you want to disable you need to:

1. Stop the virtual machine in question.
2. Locate the .VMC file for the virtual machine.
3. Open the .VMC file in notepad.
   
   You can use a different editor - but notepad is the safest as it is guaranteed to not change our file format to something that is incompatible with Virtual PC (which other editors have been known to do).
   
   
4. Find the section of the .VMC file that looks like this:
   

   <integration>
       <microsoft>         
   
           <mouse>         
   
               <allow type="boolean">true</allow>         
   
           </mouse>         
   
           
   
   Bunch of other stuff that I am skipping over to save space...         
   
           
   
       </microsoft>         
   
   </integration>
   

5. Change it to look like this:
   

   <integration>          
   
       <microsoft>           
   
           <mouse>           
   
               <allow type="boolean">true</allow>           
   
           </mouse>           
   
           <components>           
   
               <host_time_sync>           
   
                   <enabled type="boolean">false</enabled>           
   
               </host_time_sync>           
   
           </components>           
   
   
   Bunch of other stuff that I am skipping over to save space...           
   
   
   
       </microsoft>           
   
   </integration>   

6. Close notepad and save the .VMC file.
7. Start the virtual machine under Virtual PC.

Note that this will not only disable the time synchronization that happens while the virtual machine is running - it will also disable the time synchronization that occurs when you start the virtual machine.  This means that unless you are using some other method for time synchronization (network server, etc...) time inside the guest operating system will become very incorrect very quickly.

Source: http://blogs.msdn.com/virtual_pc_guy/

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