You can add disks or volumes as log in items if you go to user preferences. The Mac Mini also supports wake on demand over the network (wireless as well if you're using a Time Capsule, Airport router etc). All you have to do to make shares compatible with windows based machines over the network is check SMB in the sharing preferences panel (Click on options once in the Sharing section of system preferences).
I have a similar set up at home but my main server is my macbook pro (PLEX > ATV2) so it needs to be plugged in with the display opened to support Wake On Demand. A Mac Mini would work wonders though. It also works wirelessly for me since I'm using an Apple router (2TB Time Capsule with movies and TV Shows stored on it). I can also see my shares on our old XP desktop usually with minimal fuss. Seems to work better on Vista or 7 though.
Thanks for the quick reply.
I want to do it the other way round tho' with a windows share mapped, and always available, on a mac. I assume I don't need to enable SMB sharing on the mac to be a client?