Mac freezes on startup with vertical striped bands
That screen indicates a likely graphics card or video RAM (GPU/VRAM) failure and not a software failure. These parts are one with the logic board, so it could also be the CPU. In any event, as long as you seek Apple for repair and the mac is less than 5 years old, it might be as little as $350 to fix if it needs a repair.

You will want to back things up and see what your convenient repair options are by browsing http://locate.apple.com

If you can't get help soon or economically, edit in your Mac model year and software level (Lion, etc...)
 and we can start with some troubleshooting steps.