Another option is to figure out who makes the chipset for your adapter, go to *their* website and retrieve the drivers from them.
For example I'm using an old Linksys Wireless G USB adapter for my desktop. Linksys didn't list any Windows 7 x64 drivers for it. Did some digging and discovered it uses a Ralink chipset. Went to their website, got their drivers and BOOM! It is working like a charm.
Otherwise- what Cylphid said.