Gone. (Well, it’ll still be there if you have an earlier version of Windows installed. If you’re just running Vista, it’s gone.)

Microsoft has replaced the Boot.ini file with the Boot Configuration Data (BCD) registry file. It’s no longer a text file, so you can’t edit it directly. Instead, it’s a proprietary format like the registry.

Read this article if you want to change startup settings. If your computer won’t start, don’t sweat it. Follow these steps to run Startup Repair.
For more information, read Chapter 30 (which I wrote!) of the Windows Vista Resource Kit. Also check out the BCD FAQ and read the Boot Configuration Data in Windows Vista Word document.