I'm sorry to hear that you are having problems; however, there are plenty of Mac owners who have applied this update without any problems whatsoever - including myself. So, there is nothing wrong with this update - instead, there is some problem with your system, configuration, and/or third party software. I always follow certain precautions before and after installing any updates. For instance, some add-ons, plugins, or other third party software may cause problems because it hasn't been updated to be compatible with the newer version of the OS.
At this point, I'd suggest reading up on and booting into Safe Mode:
http://support.apple.com/kb/HT1564
http://docs.info.apple.com/article.html?path=Mac/10.6/en/21245.html
If things seem to be better after that, discard the rest of the suggestions; if not, follow these:
1. Download the combo updater from the Apple site.
2. Repair permissions in Disk Utility.
3. Verify disk in Disk Utility.
4. If you have Disk Warrior, run it from an external.
5. Make sure you have a fresh backup (bootable or Time Machine).
6. Disconnect all peripherals except keyboard and mouse.
7. If desired (but not absolutely necessary) boot into Safe Mode.
8. Run the update installer; Mac will restart when done.
9. Repair permissions.