Hi everyone! I wanted to upgrade my OS X but after downloading some 675MB, my Mac will not install the updates because of some third party software/s that I've installed. What can I do? I'm a newbie when it comes to Mac. How do I know which software/s affecting the installation. Should I delete it first before updating?

Holding down the SHIFT Key during startup, starts you into SAFE-Mode ( all third party switched off ) Then you can install without hazzle.

I tried updating on safe mode but it still won't update..

The update 'MacOSX Update Combined can't be installed. The installer could not validate the contents of the "MacOSXUpdCombo 10.5.6 package'. Contact the software manufacturer for the assistance.

What seems to be the problem?

Then, the only left choice is to backup your home folder and initialize
for a new install.

I downloaded the update on the Apple site and save it on my mac (hoping to avoid downloading it again and again). The problem now is, the download was completed but it failed the verification (check sum error).

If the Check Sum Error occurs, you have to download the update again until the verification of the check sum is okay.