You can buy it from Steinbergs website
Read the manual.
Cubase 5 is no longer sold or produced due to the release of the newer Cubase 6 and Cubase 7. These newer versions of the software retail for $400 each.
Reinstall the software
Steinberg's Cubase software can be purchased from many retailers it is not legally available for free.
Cubase is better for the beginner. ProTools is not considered a "home" recording solutions. Try the simpler version of Cubase, Cubase Essentials.
It is a music producion software and a recording sequencer.
Install software and drivers.
You have to record them yourself.
They are both good, it's more about the user and their workflow than the software.
Steinberg Cubase, with Native Instruments Komplete virtual instrument bundle.
You can get Cubase from Steinbergs website
Music Production Software