I gave up on all that USB networking stuff (too hard) and bought some NIC cards and a router. Everything can talk to everything else and do all the things I want. The modem (D-Link) handles all the firewall stuff and scores full marks on the Gibson test.
AVG seems to keep the viruses out.
A D-Link wireless access point means the laptop and the Ipaq can join the network wherever they happen to be in the house.
In the end, it was all surprisingly easy.