also.....

The USB port on the router is more than likely used to configure the router if you don't want to use the RJ45 ports and the web interface (why anyone would want to do this though is beyond me)

As Dan said, attaching the printer to one of the PC's and then sharing it is the easiest way. Or you can use a printserver but this is probably overkill for the average home setup

What make is the router?