Sibelius & Scorch
Avid Sibelius 8.7
The complete software for writing, playing, printing and publishing music notation.
Avid Scorch 1.5.x
Practice and perform music on your iPad – iTunes link
Avid Sibelius | First 8
Cut-price but not light on features, Sibelius First hits all the right notes for amateur composers and arrangers.
Check the comparison, and some more information here.
Computer requirements for Sibelius
Check out this KB article for Sibelius Frequent Asked Questions
Subelius Support pages
Sibelius Registration pages
Sibelius Documentation pages