Here’s a program I’ve known about for years now, but never ever used: clamav.
Mostly I’ve never used it because … well, after 2006, I rarely ran a Windows-only machine. And even if I did, it was rarer still that it was connected to a network. And even if it was connected to a network, it was exceedingly rare that I used it for any real surfing or whatnot.
Why? I don’t trust Windows, that’s all.
So in my case, needing clamav made about as much sense as owning a seat belt, but no car. 😐
Just for fun though, I have the screenshot above for you. I hope you appreciate it; I went to great lengths. 🙄
I have always wondered why there was no Linux distro which really only booted to a clamav scanner. Most often, when a computer is gifted/given/dumped on me, I wonder if I ought not scan for viruses, just out of curiosity.
To that end, a one-time boot USB straight to clamav in a live environment would be useful. To the best of my knowledge nobody has that. Correct me if I’m wrong.
Of course, you could just install it live in something like Linux Mint or something. Just as good, I guess.
If you want to steal my idea and start your own antivirus distro, be my guest. I release that idea into the wild, to run free and unfettered. …
Inconsolation 
I think you are right that there is no bootable clamav, but Kaspersky (and perhaps others) do downloadable free CD which boots to a live Linux-based virus scanner. Great for cleaning up Windows machines so badly infected that they won’t boot from the hard drive. Well, they probably still won’t boot after Kaspersky has deleted the infected system files 🙂
Ummm… http://sourceforge.net/projects/antiviruslivecd/
Kaspersky provides a rescue disc that runs their virus scan. The scanner itself is not open source I don’t think, but the system it boots into is some kind of Linux in a simple KDE environment. I had to use it the other day to fix a friend’s computer, and it is wonderful! The improvement in the speed of scanning is amazing when you’re not actually booted from the damn infected disk.