IMHO Windows Defender is a passive, back-end, halfhearted antivirus system which is only better than no antivirus protection at all (which is why Microsoft developed it). Microsoft updates their antivirus definitions very infrequently and only after a real virus has already started spreading. With version 10, they started to send out monthly search and destroy programs for new viruses that may already be on your system (see your Windows Malicious Software Removal Tool updates). That tool forces a check of all files on your system and is very SLOW (ever wonder why some Microsoft updates take forever to execute?).
Norton is an active antivirus system. The antivirus group at Symantec sends out updates almost daily for real and possible viruses as they are discovered world wide. Their programs check, in real time, each new file that is downloaded onto your PC and run periodic scans of all files while you system is idle so that the Norton programs don't interfere with your PC usage. Their programs also check all programs running on the PC for any virus activity they may develop while running.
I've been using Norton (PC Tools) since they first appeared in the late 60's and have never had a virus infection. I highly recommend Norton.