Macbook Pro software isn't updating properly!

I've been trying to update the software of my macbook pro for several weeks now. However, after I go through the updating process and have to restart my computer to complete it, my computer freezes at the restart page! I end up having to turn off the computer and the update never completes. Does anyone have any information that could help me?

My MacBook Pro info:

Mac OS X

Version 10.6.8

Processor: 2.4 GHz Intel Core 2 Duo

Memory: 4 GB 1067 MHz DDR3