I personally don't even use antivirus because most virus go undetected anyway. I do however factory restore my computer about once every 3 months and keep all my files and such on an external hard drive.
Not sure how you believe they MOSTLY go undetected. Once a new virus is identified it is added to the definitions file, and will generally be similar enough to most variants to at least give you a warning. Only the very latest ones will not be detected.
And do you think a virus can't attack an external drive? You are playing Russian roulette if you are not using any antivirus software. Chances are you're already infected, even after a factory restore, if you have an infection on your backup drives you are just reinfect ing yourself straight away. Not all viruses will make themselves apparent straight away, they could be date or event triggered. I would scan everything ASAP if I were you.
I had a similar infection to this last year (but with a French gendarmes warning instead of FBI) and at the same time another virus which encoded every file on my computer, my external media drive, and a media player which was connected wirelessly, but had the drive mapped across the network. Not sure how I got them, but it happened in the 30 minutes I had disabled my antivirus software while trouble shooting the PC.
Eventually managed to get everything back, but I thought I had lost all my encoded movies, music and photos. If you don't have access to a 2nd uninflected computer to download all the removal tools, you're pretty screwed.
