PowerTip of the Day, from PowerShell.com:

 

You can use WMI like this if you want to change a service start mode:

([wmi]'Win32_Service.Name="Spooler"').ChangeStartMode('Automatic').ReturnValue

([wmi]'Win32_Service.Name="Spooler"').ChangeStartMode('Manual').ReturnValue
 
Note that a return value of 0 indicates success. 
You will need Administrator privileges to change the start mode.
Thanks to www.powershell.com