Here is my only concern, PTravel: You said last time that once one PC was infected inside your firewall, the problem spread. That could be because credentials were already established with those other PC's, or it could be because there were vulnerabilities in the operating systems of those other computers that could be easily targeted by an infected computer inside your firewall. If the latter, an infected VM is just as good as any other infected computer at probing vulnerabilities.

Personally, I think that as long as you never - ever establish connections between the VM and any other computer on your network, so no credentials could be saved, you'll be fine. I would even go into the network control panel and disable the microsoft networking client on the VM just to make sure. If you have firewall software on the other "actual" pc's that lets you specifically tag the VM as "untrusted" that wouldn't hurt either, but at minimum you'll want to make sure those other PC's have their own software firewall since you will have an "unsafe" computer behind your router. Thousandth's of a percent chance kind of stuff at this point though. I think you'll be fine. Personally I'm not a fan of Microsoft Virtual PC. I much prefer VMware workstation (or even VMware Player) or VirtualBox. Once they changed VPC to the windows 7 version that it ran it's connections via RDP, it seemed far slower to me.

Edited to add: You can run ChromiumOS inside of VirtualBox or VMware. Then you wouldn't even have to have a "vulnerable" windows box. This seems pretty bulletproof and maybe not so intimidating as plain 'ol Linux.