IE9 requires Vista or Windows 7. In case you're stuck on XP but still want to view your sites in IE9 (like me): it can be done.
- download a Vista VPC image [microsoft.com] (3 or 4 files) and run the .exe. I chose the IE8 image, which turned out to require 13 GB on my disk.
- download and install Virtual PC, if you hadn't already done so. There is a link on the above page.
- launch Virtual PC, and create a new virtual machine from the .vhd you've just unpacked.
- now you need to upgrade the VM's Vista to Service Pack 2, or IE9 can't be installed. Downloading and installing all the updates through Windows Update took me about 3 hours and many reboots....
- when you've got Vista with SP2 running, download and install the IE9 beta. That's it!
You may need to allocate more RAM to the VM than the default 512 MB for the VM to run smoothly. Shutdown the VM and edit its settings in Virtual PC.