on the main Plesk 7.5 page, below the right pane where you can purchase, there is also a link to download.
Its the complete package when you download, but the default License key that comes supplied has limited capabilities. I.e. 1 client, 1 domain, etc.
But, it was more than suitable for me to evaluate the product and make my decision to purchase a full license.'
FYI: You need to have a fair amount of know-how with your server when installing. The installation itself is very painless and easy, and administering clients and domains via the web tool is easy as well, but knowing whats going on in the background and making sure that plesk isnt inadvertantly messing up something you had previously configured is the tricky part.
Like they say, best to install on a fresh machine and THEN configure all of your sites. For me, converting all of my existing websites to the new plesk system was fairly daunting, as i had to keep track of alot of things and make alot of moves and some manual configurations until I got it all perfected!