@Paul: what is your graphics card? I've run iClone 6 under Windows 7 without problems, including SpeedTrees.
I suggest you run dxdiag.exe to find out what exactly your card supports (having DX11 installed and what the card does are two different things).
Click the Display tab and you will see an entry called Feature Levels for your card. It should give you a list like this:
Feature Levels: 12.1,12.0,11.1,11.0,10.1,10.0,9.3,9.2,9.1
You should at least have 11.0. Also make sure that your on-board graphics (if any) is not interfering.