Don't know much about networking two boxes to share a DSL. But havea couple of questions and apossible idea for ya.
What do you mean by 'push' your ethernet card ? physically push it into the PCI slot, push the cable into it [rj45 connector].
now sharing files to a winXP box is fairly easy, set up SAMBA on your box - write your config file to be verry simple, workgroup and a share - giving full access, then experiment with it to tighten security.
Accessing shares from your girl's win box, again, once you have samba, you can mount her shared dir with [AFAIR] mount -t smbfs //box/Sharname /mnt/location
hope this helps.
I'll watch this thread for your answers to my above questions.