This is a cut and paste from the help file for IE7.  Just click the arrow next to the little house in the tab row when you have all the tabs open that you want for your home page and click Add or Change home page then click Use the current tab set as your home page.

To make the webpage you're currently on your home page, follow these steps.

1. If you have not already done so, open Internet Explorer and go to the page you want to use as the home page.
2. Click the arrow to the right of the Home button , and then click Add or Change home page.
3. In the Add or Change Home Page dialog box, do one of the following:
   • To start a home page tab set or to add the current webpage to your set of home page tabs, click Add this webpage to your home page tabs.
   • To replace your existing home page or home page tab set with the webpages you currently have open, click Use the current tab set as your home page. This option will only be available if you have more than one tab open in Internet Explorer.
4. Click Yes to save your changes.