I've been experiencing this problem over a year ago.
When I changed home last year, I wasn't able to connect my desktop to the router through cable, so I acquired a Wireless Adapter SMC EZ Connect G USB. It worked, but the connection was too slow. I couldn't play, I couldn't download anything (5kb/s average speed) even with good signal. I tried that adapter on my laptop, HP 6840ep with Windows 7 x64 aswell and the adapter worked perfectly. I downloaded the most recent drivers
from their site, but nothing changed. Reinstaled Windows like three or four times but nothing happened. Since I couldn't wait and needed the computer, I bought a PCI adapter later, D-Link DWL-G510 Rev. C2. Guess woot? Same thing. Slow internet, couldn't even connect sometimes, with good signal. Did the same. Reinstaled windows, downloaded most recent drivers, bla bla, and nothing. Yeasterday the adapter stopped being recognized from the system. A error "Code 10 (This device cannot start)" apeared and can't even update or install drivers now. I tried all PCI slots, I tried all USB ports, but nothing.
Intel Quad Core Q9550
Asus HD6850 DirectCU
6GB RAM 800MHz
250GB SEA + 320GB Samsung + 1TB Ext
Asus P5Q SE2