This is exactly what I need, and tragically is also the purpose that my laptop is currently serving. I'd never thought of getting a legacy system to take care of this, but that's only because I'm learning about all this stuff myself.
How to configure an $80 fileserver in 45 minutes. It'll go great with
Xbox Media Center once I'm able to stick it on the free Xbox that
rudytuesday's fiancee gave me. :D
EDIT: I totally just negotiated with my dad for a computer fitting the requirements of this little project. It was easier than I thought it would be...