[Imp]

If you are that worried why don't you just install a second operating system on a partition or second drive.
Boot from that operating system whenever you want to download anything to check it first.
Do a backup if you like first once the first ones done they are quick if set to only backup changed files.
Saves so much time if your system does go tits up, reinstalling everything is a pain but its setting the preferences up as you had them for everything that is the real PITA

My virus checker checks my emails, pretty sure my main email account the provider actually runs a virus check also.
I can then if I want to interrogate the file with a right click & another virus check before I download it.
If its suspicious as in they are not sure I can either wait for a report from my virus checker after sending them the file & or open it in sandbox mode so it cannot make any changes to my system.

Hard drives are cheap now a second drive & backup program means your covered even if your HD packs up.
If you really want to you can set up so the second drive is identical to the first rather than a backup. If the main drive fails you just boot off the other one & your back in business at the last scheduled backup point.