You are fancy here.

If you use nLite (Google about nline or search this site, I have a topic about that on AO) and a XP CD where SP2 is allready intergrate with, you can manage to have a OS of about 1 GO. I'm pretty sure that Linux distro would take aroud 1 GO of space also unless you run it a Live CD distro.

Forget AD, it take a lot of hard drive space. If you want the easy way, install XP and jusy join that machine in the same workgroup that the 2 XP workstation and created share drive.