I might recommend trying to start your computer in Safe Mode. No, seriously, even Mac OS X has a "Safe Mode".
Restart your computer, and hold COMMAND+S immediately (before the chime). Keep that key sequence held down until your computer boots completely. (When the command prompt appears, type "exit" and press enter.) If it is able to boot into Safe Mode but not regular booting, that means you have some piece of software or a Extension that is causing problems. You also may be able to see which Extension is causing problem if there is a freeze because it will be the last one(s) displayed.
If it's not able to boot completely in Safe Mode, try booting to your OS CD/DVD and choosing Disk Utilities, repairing disk errors and again to repair permissions.
-Aaron-