Unless it's a gaming desktop with a stupidly equipped external antenna you are probably asking a lot of the THG3000 router with its own internal PCB etched antennas!
It might seem a bit odd, but even turning the desktop around so that its WiFi antennas aren't blocked by its own case may help a little.
If that helps, but its still not enough, then better WiFi in the desktop is probably the next option (which will probably mean disabling the built-in WiFi). Whether you go expansion slot or USB go for a design that allows you to place the antenna away from the PC and any other metal objects.
In some circumstances power-line adapters are useful, but the longer the distance the greater the slowdown, and if you've a modern house with separate circuits for upstairs and downstairs then crossing the circuit breakers slows things down too! Keeping a powerline on the same floor (circuit) running to an access point over the area of the Desktop is an option, as would be running an ethernet cable from the router around the outside of the house and re-entering for either a direct connection to the desktop or once again an access-point.
Last suggestion would be to find and use a third party router that is renowned for it's WiFi range, but you'd still need to retain your VF router incase you ever need help from customer support!
