Does the router have movable antennas? You could try them in a different orientation. If internal, you can turn the entire router 90°.
FTR, for PCs, I prefer wireless adapters with external antennas, similar to
this. That one has detachable antennas so, with the appropriate cables, you can raise the antennas and mount high up on the wall or something. Or there is
this one that might be worth considering.
It sure would be good if you could test that computer closer to the router, however.
