Command-R to the rescue.
Just hold down Command-R during startup and Lion Recovery springs into action. It lets you choose from common utilities: You can run Disk Utility to check or repair your hard drive, erase your hard drive and reinstall a fresh copy of Lion, or restore your Mac from a Time Machine backup. You can even use Safari to get help from Apple Support online. And if Lion Recovery encounters problems, it will automatically connect to Apple over the Internet.
Les mer på: http://www.apple.com/macosx/recovery/