Share iPhone WIFI with computer over USB?
Sorry, but it's not possible to do this without jail breaking. The "Personal Hotspot" function on an iPhone only allows for sharing the 3G connection, not the wifi connection, so you can't use it as a wifi adapter. It's feasible that Apple could add this functionality, but it's a fairly niche case, so I wouldn't count on it.

More immediately, if you have a laptop with wifi available, you could use Internet Sharing (OS X)/Internet Connection Sharing (Windows) and an ethernet cable to get connectivity to your desktop. Or if you've got a Staples or something similar nearby, they usually have USB wifi adapters for $30 or less.