I know this has happened before, since there was at least one post in the MS Virtual PC newsgroup and now its happened to me.
When you start VPC, the VPC list or console shows up in the taskbar but not on the screen. Somehow the console location in the options.xml file gets some really funky numbers. Mine were somewhere in the 4million range. Changing these numbers back to 10 and 10 restores the VPC list back to your screen.
These folders and files are hidden, so you will need to go into the Tools–>Folder Options of Windows Explorer/My Computer and show hidden files/folders in order to make these changes
In Vista, the options files is located here:
Find the console section and change the left and top positions to 10 each…
In XP, the options.xml file is located in C:\Documents and Settings\%current_user%\Application Data\Microsoft\Virtual PC
%current_user% would be the user name that’s currently logged in, i.e. if you’re logged in with the Administrator account, your path would be C:\Documents and Settings\Administrator\Application Data\Microsoft\Virtual PC