Custom OS X Tiger Re-install Help

I've just sold my iMac and we're trying to figure out how to reinstall os x in order to change user info (short name, home folder, etc.) without losing the non-apple applications and media (movies, music, tv shows). Is this possible? Or will she need to make a back-up to an external drive and re-import? If it is possible, can someone possibly provide easy to follow directions in order to achieve this?

Thanks in advance for your help.