Under Virtual Server 2005 R2 you need to provide a virtual machine with specific user credentials if you want to configure the virtual machine to start up automatically when the physical computer is launched. You need to do this because Virtual Server needs to have a user context under which to access resources like .VHDs.
When you enter these credentials – Virtual Server stores this information in a secure credential store on the physical computer. The virtual machine configuration then just holds a pointer to the information in the secure credential store.
Now the problem that you can encounter is that if you configure a virtual machine to launch with specific user credentials – and then move it to another physical computer – the virtual machine will fail to launch. The reason why this happens is because the new physical computer does not have the stored credentials for the user account specified in the virtual machine configuration.
The only way to get around this is to re-enter the user credentials after moving the virtual machine.