Norton vs. Windows Defender

I have a Windows 8 PC with Defender running.  Periodically, when using some application, I'll see that some kind of "trashware" (malware/adware) is active.  For instance, inside an Excel file, there will be hyperlinks all over the place to various sites.  So, I then have to go looking for programs to uninstall to attempt to get rid of the trashware (spread by low-life trash, but I'll try not to get too personal here -- you slimebags know who you are!).  Anyway, I have in the past used Norton (including 360) for many years and was more or less happy with it, except for two things:  (1) It tremendously slowed down the PC, especially my old XP machine.  (2) I still sometimes got Adware infections that were not caught by Norton.

I was assured by the computer salesperson that I no longer need 3rd party security software when using Win 8, and the IT manager at my company agreed.  However, I'm getting this trashware. 

-- Does Norton now remove adware/spyware and so on?

-- What value does Norton provide above and beyond Win8+Defender?