Are you set to WPA security, broadcasting SSID and latest router firmware?
There is a Linksys Wireless G router in the place I'm subletting. I bought a Wireless B adapter for my PC. No problem at first. Always an excellent connection.
Then I started having internet problems. Checking the wireless icon in the tray -- it said low. The little bar icon in the "View Wireless Networks" box, however, still looked strong. At first I would disconnect from the network and re-connect and everything was great for a while (Connection indicates excellent-- easy web surfing.) Then it would happen again. Sometimes I would have disconnect-- then reboot and then connect again and it would work again for a while.
Today, however, no amount of disconnecting, reconnecting, or resetting the router restores the good connection.
CRITICAL FACT. Using my laptop int he same room there is a perfectly good, "excellent" connection. And throughout I have "4 bars" in the "View Wireless Networks" box.