USR Robotics 9108 and usb wireless 5421 really bad, help

Good Morning,
I've buyed a wireless MAXg ADSL2+ kit corresponding to a USR Robotics
9108 with an usb wireless adapter model 5421 ,
the first tests are really bad, at a distance of 1 meter I measure with
Qcheck a maximum throughput of 6Mbps while at
a distance of 5 meters on the same flat I've no signal.
Can someone tell me how this is possible and if there is a way to solve
the problem.
My real needs are more demanding, I need to serve two flats on the same
plane and other two flats on the plane above.
But first of all I can't understand why a system selled for performance
at 125Mbps delivers just 6Mbps.
Thanks

Antonio D'Ottavio
www.etantonio.it/en/