Go to Add/Remove Programs in Control Panel, uninstall 'Windows Internet Explorer 7' and reboot if asked. That will get you back to IE 6. Next, I would go to Windows Update site to see if you need to re-update any critical updates. Internet Explorer 7 may appear again as available via Windows Update, and if so, just uncheck that option or tell Windows Update (also an option) to not show you IE 7 again.
Internet Explorer 7 should work with IE 6 anyway, but I'm not a fan of IE 7... Don't like the look and feel and is not as snappy.