D-link dwl g132

Hi. I'm using an usb adapter, D-link dwl g132, to connect to an open access point. I see 3 channels available and the computer does connect but very often the connection is not stable as it should be. I've tried a lot a things, changing the settings, installing new drivers, but still many problems occur, like limited/low connectivity and stuff like that... What can i do?

How far are you away from the access point? Are there any walls/trees/houses in the way?