I Found this to be a problem recently on 10.6 & .7 & .8 &.9
I solved it by making time set manual to around February 2012
Once installed, download the proper updates from Apple Support, and install them.
Then you should be able to open the apps.
Reconfigure time set to automatic.
I Have not tested many variables, but this will most likely get you through the troublesome installation denial.
