You may sometimes see the message ‘Service is stopped’ when you try to run Windows Defender. This is usually because you are running another spyware scanner which has disabled Windows Defender. To start it up again, press the Windows key and R, then type services.msc and press enter. In the resulting menu, right click Windows Defender and then select Start. You can stop it running in the same menu.
Also, within Vista, there is what might seem like a bug -- though strictly speaking it’s an intentional feature. Windows Defender automatically blocks any program from starting automatically when Windows starts if it requires administrator privileges. That’s part of an overall policy in Vista which means the user has to manually authorise any program running with administrator privileges (which means it can carry out operations which could seriously affect your computer’s running).
In these cases, you’ll either have to get an updated version of the program that doesn’t need administrator privileges, or you’ll have to manually start the program each time you need to run it.