You say downstairs, does that mean directly under the upstairs room, so you're looking at no more than 4-6 mtrs. The problem will occur if the signal has to transmit through the floorboards that are likely to contain hot water pipes, electrical cables, etc, which all will attenuate the signal.
I'm not sure how handy you are, but you can build a high gain antenna quite easily for 2.45 GHz the frequency of WiFi click here . The idea is not new, I have designs that date back to the 1950's using the same power levels as WiFi that will transmit over greater distance than you need.
Just build it and point the antenna to were your remote computer is.