Go4Expert - Server Os's?

Hey guys, I'll make this short and sweet.

I have 2 desktops and 2 laptops.

1 desktop I want to add a server os to it to learn how to network and all that.

What free os's can I use? I used Ubuntu but I don't like how it looks like Dos. I really want to learn Windows SBS 03 but can't afford it right now. Can someone point me to a free one thats setup like sbs 03? Thanks