I'm sorry, but you'll have to buy it. This software is propriarty of Siemens and is not downloadable. You could of course use the alternative packages PG2000 software or Step 5 for Windows by IBH Softec, for which there are demo versions available. Both have been discussed here before, so searching this site will give you an answer.
If you need this for a new project, then I strongly advise you to leave S5 for what it is as it won't be available from Siemens from 2012 on. The better choice for new projects is S7-300 for which there is a free package available: STEP 7 Lite. You only have to register (free) on the Siemens site and then download and install the package.