Here is the way totest web site pages in IE7, IE8, and IE9.
Hitting F12 on IE8 it should start the Developer Tools that allows you to emulate IE7 (not IE6) using the Browser Mode. IE9 having capability to emulates back until IE7.
If you want to be 100% sure to emulate the old IE browsers you can use VMs provided by Microsoft (incredibly they are also provided for free). As a side note, keep in mind that testing on VMs is more time consuming than using IE Developer Tools, IMHO VMs testing might be worth when testing something that’s JS/CSS greedy like a web app or a complex website, not for simple sites that use jQuery and some CSS.
IE Tester It does IE6, 7, 8 & 9
You can also use the browser shots website if you are testing layout. You put in the url and it returns images of your site in a bunch of different browsers.