MacBook freezing

You should probably do a fresh install of the OS.

1. Repair the permissions for your disk's files. Occasionally the user permissions associated with files or applications are set incorrectly. Try verifying the permissions using the Disk Utility application, which is located in /Applications/Utilities. In Disk Utility, select the startup disk in the left column and click First Aid. Click Verify Disk Permissions to test permissions, or Repair Disk Permissions to test and repair them. It's a good idea to repair disk permissions as a regular maintenance task after upgrading or installing new software.

2. Maybe Disk Utility can repair the disk this way:

-Search your installation DVD's.
-Insert the disc 1 and sthut down your Mac
-Start your Mac again while holding down the C key
-Now your Mac should boot from the CD, this can take some time.
-After a while you will see a installation screen where you can chose a language.
-Chose English and continue.
-Then click some time's next and agree with the "Software Agreement".
-Open Disk Utility. Top menu bar --> Utilities --> Open Disk Utility.
-Select your hard disk in the list of disks and volumes on the left side
-Go to the First Aid (tab somewhere on the right).
-Click the button to verify the permissions.
-Then Click the button to repair the permissions.
-Click the button to verify the disk.
-Then Click the button to repair the disk.
-Exit the Disk Utility and the installer.
-Restart your Mac

When your computer "gives up" and tells you to restart, are you seeing a grey screen with the instructions in several languages? If so, you are getting kernel panics. As Haribol says, repairing your disk permissions is a good first step; this is something that should be done regularly, especially after installing new software.