How to Start Slide Numbers at Zero

On the File menu, click Page Setup.

In the Page Setup dialog box, in the Number slides from box, change the number from 1 to 0, and then click OK.

Your second slide will now be numbered 1. (Doing this could be a good idea if you've chosen not to show numbers on the title slide.) You might want to change the starting number back to 1 when you print handouts, notes, or outlines, though.

Note: There are other options here that might interest you, such as those under Slides sized for. If you're printing your slides, these choices come in handy.