Are you connected to your router by an ethernet cable and trying to set up your wireless connection? In some PC's windows will automatically disable one or the other but not all and there may be conflict. If this is the case remove the ethernet cable.
Could it be that your PC is quite a way away from your router and needs a booster?
I assume that your antenna is securely connected.
It may be that the network your PC is trying to join is not broadcasting its network name (SSID). Should this be the case:-
Open Connect to a Network by clicking the Start button
Click Set up a connection or network.
Click Manually connect to a wireless network, and then type the network information.
The network will be added to your list of networks and will be available to connect to
in the future when your computer is in range
of the network.
The only other thing I can think is the wifi frequency of the router needs changing. You will have to go into the settings of your router to change this. Channel 11 is usually a good bet but if you do change from whatever you have now then change it to something at least 3 points higher. To access these settings you will need to type in your router address - usually 192.168.1.xxxx
just a thought, but what standard does the wifi use g/h or the newer n.
my belkin wifi adapter is N but when installed initially (n) is disabled by default and signal strength show only about 2 bars even if I am only feet away from the router. maybe worth a check .