If you want a simple solution then download Microsoft's Windows Media. It's free (as long as all your existing MS software is genuine) and will rip your cd's and create a library for you. Once done you will then need to tell the Squeezebox Server (running on your laptop) where your new music library is stored (e.g. C:\My documents\My Music) in order for it to be able to stream it to your Duet's receiver.
If you want something a bit more advanced but still free then try Foobar 2000 (note all codecs have to be downloaded seperately and stored within its components folder for Foobar to use).