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Once you're on the site in question, go to View -> Show Page Info, click on the Preferences tab in the panel that appears, and therein click the Other tab.

Just realize that a lot of these sites block non-IE browsers because they actually require IE-specific features. ActiveX, VBScript, whatever. Not that that's excusable or remotely reasonable... I just wanted to point out that all this will do is make the server think you're running IE so it won't block you. Beyond that, it won't "emulate IE".