I use Avast! Anti-virus, and Malwarebytes Anti-Malware. They are both free to home users, and I put them on every PC I fix/upgrade/build, except for the PCs where the user wants to use a certain anti-virus product (usually Kasperky, which is admittedly excellent). I really recommend you uninstall Norton and put Anti-virus, and Malwarebytes Anti-Malware on instead.
Avast! and Malwarebyte Anti-Malware also co-exist well together, which isn't something you can count on with many anti-virus programs. Avast! runs in the background, constantly checking for infections, whereas MAM has to be run by the user to check chosen files (in the free version, anyway, which is all that I use).
Chrome (the browser I mostly use) sometimes thinks a file I've downloaded might be infected, and so doesn't make the file available. In that case, I go to Chrome's menu, select Downloads, and on the Downloads menu I select 'Keep'. At that point, I'm almost certain the file is clean, since if not then Avast! would have detected and deleted the file, but since it's not, then I check the file with Avast! and then MAM, and if they say it's clean then I accept the file as being clean.
If I download a file that is infected, then Avast! usually detects it and stops the download. Offhand, I can't remember once when this hasn't happened.
