IE 7 and 8 are not available in the installer; I'm running Win7 x64 with IE 9.0 installed. Falling back to XP Mode for now.

So if you go to Tools > Developer Tools the browser modes for IE 7 and 8 are not there?

The IE7/IE8 compatibility modes are notoriously unreliable, e.g.:

Most importantly, why does my page not work in IE7, even though it works in IE7 mode in IE9?

http://stackoverflow.com/questions/5993222/why-does-my-page-...

IE9’s emulation of older IE document modes makes this easier but those emulations are not exact.

http://blogs.msdn.com/b/ie/archive/2011/02/04/testing-multip...

IE Collection is great on Windows. I recommend it and use it often.