Can Worms/Viruses Traverse Subnets?

While I'm confident that my wireless network is safe from outside intrusion, when I do give someone access to my network I wonder whether I'm exposing my other computers to any viruses/worms that they may have on their computer. If I were to split my network into two subnets (trusted and untrusted), will this keep my compters protected?

splitting it and having a decent firewall or 2nd router in place on your private network should keep any worms at bay.

but yeah, worms can get acrossed subnets, they spread on the internet if they couldn't.

Yes there are many that can spread across networks. Plus a computer directly into a broadband modem with a public IP address for a while, or DMZ a computer. No antivirus, no Windows updates...and see how many DCOM vulnerability bugs you catch.