Spocksmum - getting VM involved would be extra expense when not needed. If the 2 desktops are in one room then as Cindy34 pointed out, you can get a HomePlug/ PowerLine unit (they are just alternate names but the technology is the same) with between 1 and 3 ethernet sockets on, so if both downstairs machines are in 1 room, and close enough (and you don't mind a bit of cable from the mains socket where the HomePlug is in), then a 2 or 3 socket unit would work fine.
Alternatively you could use one of the adaptors which has a wireless router inside, so it gives your downstairs room(s) a strong signal, and then links to one with an ethernet cable plugged into the VM router. With the Wi-Fi version you'd need to do a bit of setting up (to give it an SSID {network} name and wireless key {encryption code}) so no neighbours would be able to connect via your VM box, and so the downstairs PCs would "see" it's signal and connect to it.
Sometimes when setting up a wireless unit you have the choice of which channel number (1 to 11, from memory) to use. There are a few utilities for smartphones, and someting called InSSIDer for desktops which can list the different wireless networks (yours and your neighbours) so you can choose a channel number to give least interference to/from others.
If you are not using the wireless feature on your VM box upstairs, you could even disable that wireless and allow the downstairs one to be the only SSID network in your home. Reason I mention this is that some network devices would simply reconnect to your upstairs VM router even if you had used the same SSID on a device 10 feet away in the same room as the desktop so it would appear as if there was no difference possible...
I have to say that in a room where I had a choice, wireless would be second to cable (but having said that, a lot of people don't like having cables around the skirting boards, etc, etc, however neatly they are fixed / painted over / hidden). Wireless can give slower connections than cable (in part because only one device is allowed to transmit at a time, whereas with cables all devices can transmit and receive simultaneously).