Astonishingly, some sites reject Internet Explorer 7 as out-of-date, but they accept IE 6. (Lazy coding, I suppose.) Microsoft has a utility to make IE lie about its version, but it runs as a wrapper around IE, setting the UA in the registry, running IE, and then setting it back when IE exits.
There is a spiffy browser extension that lets you change the user agent on the fly. See http://www.enhanceie.com/ietoys/uapick.asp
While you're there, he's got other good tools, like SlickRun.