There’s two ways that you’ve got the option to disable tabbed browsing.
If you are a system administrator, here are some registry hacks to Enable/Disable Tabbed Browsing.
Just unzip, and double-click.
Disable Tabbed Browsing in Internet Explorer 7
Why you'd want to disable tabbed browsing is a mystery to me, but I received a request for a registry hack to force IE7 to not use tabs.
