Frankly, the usual question is how to program the Palm to fetch Internet material and not how to use devices on the PC.

But you ask how to use the modem in the PC from the Palm. That takes software like MochaPPP (example at http://www.win-hand.com/help/server/ ) which is some sort of middleware that runs on the PC and arbitrates the requests.

PalmSource and PalmOne have developer links that you can start to research and you can look at ROMTRANSFER which is about all that PalmSource reveals on how to control/talk across the USB port.

If you are not going to write your own software, then I don't see a ready to use solution.

Bob