If Twitter was indexed, then I could count on brief tweeted solutions reaching a public audience, but they're not. So here's a very brief post, with perhaps many more to come, giving a solution to a tricky problem. I hope that it's reliably indexed and found for those experiencing the same issue, even if that audience is very small, solutions deserve to be shared! ;)
There is an issue with the WUSB54G network adapter on Windows 7 (on Vista as well I think). It could only be present on a 64-bit OS, I don't know. The Wifi connection will repeatedly drop, and many times the adapter will completely disappear from device manager, as if it has been unplugged. The only thing to be done is to unplug and replug the adapter.
It seems like a power management issue, but the Power Management tab on the adapter properties in device manager doesn't help. Nor does adjusting USB Selective Suspend in Power Options.
The solution is to install the RT2500USB driver from Ralinktech.com
Your device will now appear as an RT2500.
02-01-11 Update: I discovered the true problem was, as might be expected, insufficient power reaching the USB device. It didn't help that I was using a long, light-up, cable, plugged into the front panel of my computer. I moved the plug to a powered USB Hub, reverted the driver, and have had no problems since.
If still having difficulties, try running WLAN Optimizer, perhaps even in streaming mode. Vista was notorious for a terrible background scan lag spike every 60 seconds, which while drastically reduced in Windows 7 doesn't seem entirely gone.