MAC OS X Reinstall Help

Hello,

I have a MAC laptop running MAC os X.  I was performing an update and the laptop froze.  Now when it reboots it hangs on the MAC os X screen during bootup and does not completely load.  In an attempt to fix the problem I placed the OS CD in the computer and booted to it.  I performed a VERIFY and a REPAIR and this did not fix the problem.  I still hang on the MAC OS X screen during boot.  How do i fix this?  Is there a different tool that i should be using?  It is now my understanding that I need to reinstall the OS.  I have alot of important data and work files on the laptop.  How can I back up the data before reinstalling or how can I reinstall without loosing the data?

Thanks