Internet Explorer 9 - Early Preview Version released for Vista and Windows 7
The preview version is primarily targeted for developers to get an early start in supporting this new framework. This very early build is not fully functional yet and it is only applicable for Vista SP2 or Windows 7 test PCs.
IE 9 - Platform Preview Version
QUOTE: Microsoft yesterday unveiled a very early edition of its next-generation browser, IE9. So early, in fact, that it's more "pre" than pre-release, pre-beta or even pre-alpha. Can I run the preview in Windows XP? No - IE9's graphics processor powered-acceleration requires Direct2D and DirectWrite, APIs created for Windows 7, then back-ported last October to Vista Service Pack 2 (SP2).
Official Download site