Upgrading OS unknown issue

I have two 2006 imacs both intel based that have 10.4 loaded. I wish to upgrade to 10.6.8 Snow Leopard. It starts the installation just fine on both machines then half way through stops with an error message that "some files could not be installed" Restart and try again. After numerous restarts it happens each time. I have successfully installed the 10.6 on several machines in the past as old as these but never encountered this issue. What is going on and how can I get around this?