Unless you run IE 6 in a virtual machine session (or follow one of the dubious Internet hacks), you cannot run IE 6 and 7 at the same time. Alas, many users and administrators will install IE 7 first, before testing, only to find out that one or more applications don't work with Microsoft's latest browser version.
Fortunately, uninstalling IE 7 at the desktop level is pretty easy. Just go to the Add/Remove Programs Control Panel applet and choose the Remove button to the right of Windows Internet Explorer 7. After an uninstall and reboot, the computer should have the previous version running. I've done this many times and never had a problem, although I'm sure it will hiccup on some small percentage of folks.
I'm not sure if I can blame Microsoft for breaking so many applications in the pursuit of better security. The beta versions of the software have been available for nearly a year. If I've been able to test all my applications by now, certainly your vendor should have been able.
If your application doesn't work with IE 7, you can always stick with IE 6, or use another browser, if the application supports it. I'd recommend that you quickly test all your applications and needed Web sites for IE 7 compatibility and request IE 7 compatibility if it isn't already supported. I suspect come Nov. 2, we'll all be reading about some unhappily surprised end-users. Make sure your company is prepared.