Windows update service can be started from command line(CMD) using the below command.
net start wuauserv
Update service can be stopped using the below command
net stop wuauserv
Note that on Windows Vista and Windows 7 machines net start/stop commands should be run from elevated administrator command prompt. Other wise you will get the following error.
C:\Users\administrator>net stop wuauserv System error 5 has occurred. Access is denied.