There was a pastebin fairly recently that suggested exploiting the fact you can attach short amounts of data to the blockchain by spamming the network with transactions that contain signatures for random viruses. Thus being flagged by tons of AV software, and potentially causing a loss of coins.
http://pastebin.com/ct2WHUK5The good news is that you can't really create a virus via the blockchain. Messages are limited in size (20 bytes? I think), and there's really no room to create an exploit since the format is so well-defined.
Is this info legit?
If so, sounds like fun.
I quit running antivirus years ago because of issues like exactly this. It causes more problems then it solves.
I run my web browsers either sandboxed or in a virtual machine. Run any suspicious software (like keygens) sandboxed or in a VM, and only download software that I'm relatively sure is safe.