Virtual PC should emulate viruses just the same way it emulates everything else. But since Virtual PC operates within its own disk image, it shouldn't mess with OS X at all (unless, perhaps, you have Shared Folders mounted).
You should follow many of the same security measures in an emulated Windows environment as you would in a real one. Most viruses and malware on Windows spread through email or the web (well, really just IE). It's possible you wouldn't be doing either of those things in VPC anyway, in which case you wouldn't need to worry too much. You don't even need to give VPC network access if you don't need it.