>>>I need to go to the Command Prompt on the customer server.
>>>I am connecting (via RDP) to the customer server. For some reason, the Windows button (the one in the lower-left corner) is disabled. When I click on it, nothing happens.
>>>Any other way I can open the command prompt (other than Windows -> Programs -> Accessories, etc.)?
>>>
>>>TIA
>>
>>Windows-key + R will bring up a prompt to run a file. Type "cmd.exe" and see if that works.
>
>This (Windows-key + R) brings up a prompt on my computer, not on the RDP. But what Hugo suggested, works.
>Thank you.

Ctrl+Alt+End passes along Ctrl+Alt+Del to the remote computer.

You can use the Alt+Home key to use the Windows key on the remote computer. So Alt+Home then R should do it. Haven't tried this, but it's what I read online.