Windows Defender and Norton?

I'm sure this has been asked  before, but I couldn't find anything in the search.

 

I have Windows 7, 64 bit. I'm running Norton Internet Security 2010.

Can I also turn on Windows Defender?

 

If yes, I would have to turn off the real-time aspect of Windows Defender, correct?

 

Is Windows Defender worth it if I'm already using Norton Internet Security 2010 and the free version of MalwareBytes?

 

Sorry, new computer, and I want to keep it safe!