A kinda rule of thumb indoors is 50ft or 3 walls for reliable reception.
According to DLink any angle of the computer/router to the walls is quite important otherwise the walls appear thicker to the signal than if it was a direct line.
You can take steps to optimise your reception and avoid interference.
1) raise the router above furniture level
2) Experiment with channels (some will work better or worse depending on your environment)
3) If you can detect strong neighbouring wifi, use a channel 5 stops away from strongest.
4) Relocate cordless phone base or video sender etc.
5) Be prepared to move the computer (or at least turn it so your body is not between the router signal and the wireless adapter's antenna.