Actually, I don't think Cubase has anything shareware... they've got demo stuff but you'll get no save option etc... and unless you're upgrading from a previous version you're looking at hundreds of dollars.
Right now the best deal out there is Pro Tools Free. It's a complete midi sequencer with 8 tracks of audio, nothing disabled and totally free. It's available on the web.. I don't have the link but just go to google.com and do a search...there's pc and Mac versions avail.
You're gonna need a half decent computer to run it too.. I use Mac and it's something like a recommended 266 processor, 200mb ram and a G3 or very close to it.
If all you wanna do is make MP3's though there's tons of shareware things for that... all you need is a minimum PPC.