If you can use a null modem cable, then you don't need PPP or anything. You just need a terminal emulator in DOS (Norton Commander has one). Setup is very simple too.
You have to edit your /etc/inittab file to allow login over the serial line. You will have to run an mgetty or agetty on a serial line. You can consult the Serial howto. In this way, not only you can copy files, but you can run a full (text mode) Linux session from that laptop. If you need more help, mail me personally as I don't read the list often (I'm not even a Debian user right now).
