[Aragorn]

The most common cause of wireless signal problems is interference - other stuff using the 2.4Ghz band, the microwave being the obvious.

Most wireless cards will allow you to change the channel, so try setting it to something like channel 1 or 10.

Alternatively, invest in a wireless router. They tend to be more robust than ad-hoc connections and you won't have to leave to downstairs PC on while you use the upstairs one.

HTH