Most freezes that occur between the "Welcome to Mac" screen and the actual loading of the finder are caused by: extension and control panel problems and conflicts or damaged or disabled system files. One way to verify that this is the problem, is to boot with extensions off.

If the machine boots properly with the shift key down, you know the problem is isolated to a control panel or extension.

If the computer freezes while in a program or while exiting a program try force quitting. To do this, hold down the Option, Command, and Escape keys. This doesn't always work but it is worth a try if you have a file open in another application.