Tools -> Internet Options
In the 'General' Tab, click on 'Settings' in the Tab field.
In the 'Enable Tabbed Browsing', check the box that says 'Open Home Page for New Tabs instead of Blank Page'
BTW, I recommend that all IE7 users check out the IE7Pro add-on (free) - it adds an immense amount of functionality to IE7 including a ad/flash-block, user scripts (for Google, MySpace, saving FLVs from video-sites like Youtube/Google Video or you can make your own), mouse gestures and other fun little things.