Initially I would recommend to relocate either the PC
or the Wifi Source.

Else...

Buy a cable,
Firstly measure the distance between your router and
The computer upstairs. (I assume it has atleast
2 ports for Cabled Lan) Go to a hardware store that
sell UTP Cable and connectors (Lan Cable) and let
them make it for you. Here is a reference for your
guidance on color coding of a cable...

http://www.whoknowstech.com/images/cable_utp_ct-568-a.gif