I bump my topic.
So I'm having this issue on 2 laptops (2 macbook pro) with 7x64 SP1, and also with my bro's main computer 7x86 without SP1 (classic computer).
Only my old Fujitsu laptop with 7 (no SP1) will run horizon.
It can be stupid, but the only thing all the "non working" computers have in common is the NVIDIA driver ( 266.58 )
Here is a screenshot .NET 3.5.1 installed and .NET 4 as well (it's not working with and without .NET 4)