IE8 is the default browser for W7. All you should have to do is uninstall IE9 through your Control Panel. It's located in the updates section of the Programs and Features section. As long as you are 64bit and not just 32bit, it should be all you need to do.
I agree with what you said but you forgot to add that uninstalling IE9 will AUTOMATICALLY reinstall IE8 ....
All the best Brett