Of course you can connect 2 computers via a serial cable using
ppp, but, in this instance, why? With such a small distance
between the 2 computers, use network cards, ethernet, and a 5-port hub.
You'll get _much_ faster speed also.
After you get some experience with Linux, you will want to
move the internet connection to your linux box, and have it
serve the WinXP box.