I have seen other people have similar wireless problems, that were fixed by getting a new router under warranty....(assuming all of the things we have tried here failed)
the absolute LAST thing that you can try, is to have another wireless machine (laptop/desktop/whatever) placed right next to the machine that is having trouble, and try connecting....
at least if that one also doesn't work, then we can say it is the router....
if it does work, it could be the pci card in the desktop.
see if you have a buddy with a laptop setup for wireless, and have him come over and check it out.
if you have access to a different wireless PCI card, or a wireless USB, give that a shot too