bill - 1:25 am on Mar 12, 2010 (gmt 0)
The best way is to run it in a virtual machine. You didn't mention which version of Windows 7 you were running so I'll list up the options. ;)
You must have Windows 7 Professional, Enterprise, or Ultimate to run Windows XP Mode. That's a built-in version of XP that you can run within Windows 7. You could install IE7 in there, add the link to your desktop, and run IE7 and IE8 side-by-side.
If you're using a lesser version of Windows 7, then you'll want to look into Microsoft's Internet Explorer Application Compatibility VPC Image [microsoft.com]. Microsoft provides these working images of XP SP3 with IE6, IE7 & IE8 for testing purposes. You'll need to run these images in Microsoft's VirtualPC which is free. It's not quite as smooth and integrated as XP Mode, but you'll be using the same technology.
If you're the do-it-yourself type then pick up one of the various virtual machine softwares out there.
You'll need another copy of Windows to install in the virtial machine. Then download and install the versions of IE you want to test.
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