You will only get a virus over the network, "wired" or "wireless", if an infected file is transferred over the Local Area Network using "File & Printer Sharing". If you don't have a need to transfer files between computers, untick the box in the Wireless Network Connection's Properties for "File & Printer Sharing for Microsoft Networks".
<<< is there a way to stop my connection slowing down when somebody else on the wireless is (e.g.) downloading something? >>>
It's not the wireless connection that will be slowing you down but the 1Mbps broadband connection that's being shared by whoever is online.