Network card not good enough?



The pc and modem are at seperate ends of the house, its a relatively small bungalo and i get 2~3 bars signal strength usually. There are no thick walls or anything so im not sure
 


In my house, if I go upstairs directly over the router, I get almost full speed over WiFi. Walking 15 feet down the hall, it decreases 25% or more.
 


Once or twice I was actually able to get 15 down on my pc though.It seems like some nights its worse than others, seems like something else to me
 


What happens with the PC right next to the router?