Say if you let a virus "in" by actually clicking on the executable and running it, are you pretty SOL? Or if you ran a Virus scan with Norton 360, will it get it, and all will be better?

Or , if the program/virus is already Executed, you're pretty much done for entirely?

Reason I asked was, apparently I had a virus running in the background (and Norton didn't even find a virus when I did a full comprehensive scan)...and the virus was STILL there , running in the background.

Any reason for this?