not quite like sibelius, but there is finale notepad that u can download. it is a bit slow, but hay it is free.

Please don't the computer start doing the song writing too! AHHHHHH!!!

(Look, I am a computer professional, but when computers start to write songs, it's really gone too far!) I have played with some virtual composing software, and it can follow your leads even, but let's face it, leave the composing to the professionals not the computer!