I have tried 5 different wireless USB adapters on my PC, all ranging between £15 and £80, they all state Windows 7 x64 compatible, but none of them worked properly. The message comes up saying new hardware detected, yet the adapter can never get a good signal on any wifi networks, even though my tablet and laptops would get 100% signal bars on those same networks. I have tried various USB ports (all USB 2.0) Luckily I have been able to get refunds on them all, but really, I am now thinking there is something on my PC interfering?
Most devices with built-in Wi-Fi (espcially laptops) is able to design nice big antenna(e) in to the device to ensure good singnal quality.
Theoretically, you'll need an antenna that is at least 6.25cm long to have good reception on 2.4GHz. The minimum physical possible length is half of that, which I guess is what is being used in your USB dongles.
Also, many built-in WIFI uses MIMO, which greatly boosts signal quality, but requires multiple antennae to be at a certain distance and angle which cannot easily fit into an usb dongle.
This sounds like a driver or software issue... Without knowing you configuration, hardware and software it sounds like the wrong driver is associating itself to the adapter.
If software is involved it could be a simple hkey issue with x64 software. If installed the x64 version Windows will still look in the x86 location of hkey and if not found there it will skip the step and act like everything is working well but not start the software.
It could be another program like motionJoy that automatically associates certain items with their program and install the wrong driver when the new device is found.