Check to see if there are headers for extra USB ports, and get a rear
plate for it.
Get a cable/DSL modem with ethernet, and a PCI NIC, or a PCI modem if
it is dial-up.
USB is host PC to whatever, not the other way around.
The usb-to-parallel adapters are only for hooking a parallel printer to a USB
port. Won't work the other way.
You might consider getting a USB adapter for their PC. They are relatively
cheap ($15-$20) and would provide extra USB ports for other things as well.
install a network card for the cable modem