How do you re install an apple laptop?

Your Mac should come with a Recovery Disk that enables you to reinstall Mac OS X. Simply insert the DVD when you boot up and hold the 'C' key (this tells the computer to boot from the DVD) and follow the onscreen instructions.

Reinstalling takes a bit of time, so perhaps it's worthwhile to try out some other options in the recovery disk, such as Disk Utility to try and back up whatever data you can, or troubleshoot problems that may not require a fresh install.