I want to run cmd.exe as administrator with arguments from C# in order to prevent a UAC popup. This is necessary in order to use it as an automated installation
There is at least one problem with your command, this line:
Arguments = "/user:Administrator cmd /K " + command
Should be:
Arguments = "/user:Administrator \"cmd /K " + command + "\""
Also, this won't work as a fully automated process, because it will ask for the Administrator's password which in Windows Vista and newer is not known.