Yep very doable. you would need to install the squeezebox server onto the dell computer, connect it to the router and give it a static IP address in the same range as your existing computers then setup squeezeserver to use the usb hard drive as it's music folder and set the squeezebox to look at the IP address of the dell for it's music.
For the mac to be able to see the drive you just need to share it on the network.
I personally have done it using a version of linux installed on the computer and then installing the squeezebox server, but it can work equally well running windows and the windows version of the server.
http://www.logitechsq...queezebox-server.html