You can share the entire drive (at least in XP Pro -- enable sharing in the Sharing tab of the Properties menu for the root directory of the drive), and then attach to it as a network drive in OS X. I do this to back up my iTunes library.
Unless you actually mean something else... Any time you mount a network share it looks like a drive to OS X, so you don't even have to share your entire drive to make an XP box look like and external ethernet HDD. FWIW XP Pro actually creates a default hidden "administrative" share for each HDD in your machine \\machine\c$, and you can access that with an administrator's username/password.
If that's all you need your XP box to do I'd suggest you look into something a bit more lightweight and/or Mac friendly, i.e. use a Linux distro and NFS or even Darwin instead of XP.