Last night my Linksys WUSB600N stopped connecting to my router (Linksys WRT400N). Over the past week or so the wireless connection would cut out for a minute and then reconnect, which it had never done before, and then yesterday it stopped connecting all together. I'm running Windows 7 Ultimate x64. I have the latest drivers and my router has the latest firmware. Linksys tech support is just a bunch of idiots BTW. Anyway I brought the adapter to work with me today and hooked it up to my laptop and it works just fine. Linksys offered to RMA it but I don't know if anything is even wrong with it. I even reformatted my comp last night (which I was planning to do anyway) and the adapter still doesn't connect. It sees the networks and asks me for my security key but it just fails to connect. Every other computer in my house connects to the router just fine so I'm lost.
I hope you are using WPA2 authentication and you are entering the correct passcode. If not, it is quite possible that the USB wireless is connecting to an open network in the neighborhood.
I had a similar no-connect issue with an older wireless card, and when I checked, I realized that this older card did not have WPA2; instead it had WPA, and my network was setup for WPA2. Replaced this card with a card suitable for WPA2 and everything was fine.
In your case, you may have a wireless USB issue. Time to RMA for replacement. BTW, I am not a fan of wireless USB - prefer PCI card instead.