I have VPC running on my PB. TBH, I can't be bothered to download all the security updates and service packs. I will only be using it to run the odd bit or software that has no Mac alternative, i.e. Programming Philips Pronto remote controls and Lutron Homeworks lighting systems.
It doesn't have access to a shared folder on the Mac. So nothing can get onto the Mac file system. Everything is saved to a USB key anyway. I have Virex installed which will probably pick anything up. I can't be asked to install AV software as VPC is slow enough anyway.
I won't setup email and will do only basic web browsing on VPC. I be pretty safe.