Installing Cubase 6 is impossible.

I work Under Windows 7 Professional 64 bits. I used to have Cubase Studio 5 and it worked fine. When I upgraded to Cubase 6 the problems began. It simply will not install. Every time I try to install I get an error message. What a piece of shit. All I want is to go back to Cubase 5 Studio. Unfortunately I can’t because the Dongle will only allow Cubase 6. How can I modify my account so that I can downgrade back to Cubase Studio 5? Thank you. Thank you.