If there is an infection on the machine or some protection suite then installs can fail. This is where I suggest a scan with the software titles that Grif notes at:
It's also known that we have to uninstall old versions before installing the new ones. Sometimes that's the only way.
Zone Alarm has been asking me to download their latest version for some time now. I have no difficulty downloading it but it will not install always failing when it gets to 15%.
I have never had any trouble with their updates previously.
My wife's laptop which contains the same security programs installed it without any difficulty.