The powerbook can network just fine, as pointed out already it has a network port (100baseT) you could just plug this into any PC with a short ethernet cable) The problem is that your PC, (according to your post) lacks the built in networking ports. I don't think you will find a way to setup a newtwork without upgrading the PC.

Are you trying to move lots of data or just connect to a dialup internet service?

Is the modem you are conecting to a DSL modem? if it is... it should be possible to have the PC pass the network traffic from the USB DSL modem to a network port (ie act like a bridge) except you dont have a network port on the PC, and this setup is diffucult for experts to setup.

You can get a new DSL modem that could be shared by both computers , if you ISP allows it.