do i have to install new definitions?

if i dont, the icon keeps coming up every time i turn my computer on

i already have norton


what to do?
if i do install
what happens


cause this is the first time i even knew there was this in my computer

if i install, what will happen

will my computer go back to how it was when i didnt know about this defender?

Its best that you update windows defender anyway should something happen. Most people don't recommend using Nortons though.

Windows Defender is an anti-spyware. Norton is an anti-virus.
For best security, you need to run both anti-virus and anti-spyware (firewall too).

Windows Defender updates using Windows Updates, so if you have our Windows set to update automatically (and you should), Windows Defender should update its definitions automatically.