Possibly a really stupid question, since I don't really understand how wireless works (though I have a wireless network at home).

I have an AV receiver that has USB 2.0 input. I use it to play music files from my laptop, running Foobar 2k, connected by USB cable. The laptop has built-in WiFi.

Is there any chance I could make this setup wireless, by plugging something like this:

http://kbserver.netgear.com/products/wg111v2.asp

into the AVR's USB port and creating an 'ad hoc' network consisting of the laptop and the AVR?

Somehow I doubt that will work. The wireless adapter is likely to require drivers, either supplied or usually built in to the host OS. Your AVR is unlikely to have that capability. However, it might be a good idea to call up the manufacturer's tech support line or do a Google search first.

Doesn't work if your stereos appear as a sound card on your pc.

When the AVR is connected to the laptop, a Windows 'USB Audio Codec' appears as an option in menus for choosing an output device for Audition, Foobar2k, etc. It disappears when the AVR is disconnected. Is that what you mean by 'appears as a sound card'?