Viruses crossing different LAN workgroups – how likely ?

How dangerous is it to put a PC (running XP) that is suspected to have a virus, perhaps even a rootkit type, onto my LAN network when my network workgroup is called “fred” and the suspect PC’s workgroup is called “alice” ?

Note I would check that netbui is not enabled, as I know this protocol can traverse workgroups.
Also no workgroup “bridges” exist.

btw – I ask as my router does not offer a DMZ and I often need to use facilities such as Trend Micro’s “Housecall” (which I find very useful, even if just to identify a problem)

Regards all – John in the N.W. UK