(That is the Windows command key near the bottom left of the keyboard pressed at the same time as the 'u' key)
Follow these instructions from woutware
(He's also got a link to a Youtube video in case you're having trouble)
which are basically:
* Insert the original Windows Server 2008 installation DVD.
(You'll need to do this in the Hyper-V settings to allocate the drive to the guest OS)
* Reset the computer, boot from DVD.
(watching for the press a key prompt)
* Choose option "Repair your computer".
* Start command prompt.
* CD C:\Windows\System32
* MOVE Utilman.exe Utilman.exe.bak
* COPY Cmd.exe Utilman.exe
* Restart windows
* At the login screen, type Windows-U, this fires up cmd.exe now
* NET USER administrator
* Presto! Login as administrator!
* Now repair computer again and move the original Utilman.exe back.
Now if your server is virtualized with Hyper-V you'll be connected via Remote Desktop or Hyper-V Manager. Pressing Windows-U starts the accessibility program on your local PC not the remote server session.
The trick is to run the remote session in full screen mode (note this is different from maximised). On Hyper-V Manager you can get that option from the View menu or you can press Ctrl-Alt-Break.
In full screen mode, the Windows-U command will be successfully passed to the server and you can complete woutware's instructions to load a command shell and use the NET USER command to reset the password.