I just bought a new HP dv6 notebook, It came with windows 7. I was offered Norton free for 60 days,. If windows Defender is a part of windows7, is it necessary to install Norton?--- And, Is windows Defender one of the more dependable anti virus programs available as it stands alone on my computer?
Microsoft Security Essentials has replaced Windows Defender. It's a good option in Free AV.
Other good options are AVG or Avast Free Antivirus.
Use those AV programs together with Malwarebytes Anti-Malware for a well rounded security setup.