Connect Dell computer to Airport Extreme

An Airport Extreme provides a wireless connection to the internet. The
signal is at full strength on a Mac purchased in January 2007. A new Dell
laptop shows the wireless signal strength to be 2 bars out of 5. The Dell
can connect wirelessly to the system that the Mac uses but cannot access
the internet wirelessly, a box says that the signal may be too weak. The
Dell can access the internet if attached to the Airport by a cord. I
reconfigured the Airport using Airport Administrator on the Mac, but
neither changing the channel not using the X security setting worked.
Any suggestions?