Are you running 10.8.5?
If so, did you delete the "mach_kernel" file that appeared after the 10.8.5 update installed?
Yes, running 10.8.5
and I think yes... I remember removing mach_kernel.
Is this the issue?
Ok, yes it was. Thanks!
What is in mach_kernel to prevent the install?
"mach_kernel" is the base file for OS X.
If you run Time Machine and have backups prior to the 10.8.5 update, you can restore it.
You'll need a preference pane called "Secrets" to eb abel to see it. You can get "Secrets" here: http://secrets.en.softonic.com/mac
Install it, and open your System Preferences. You can make "hidden files" visible, even in Time Machine, and then you can resotee the mach_kernel file to its original location.
If you DON'T use Time Machine, there's a zip file I have with a replacement here: http://tinyurl.com/kq3cu72
Click it and it should download.
Put the zip file in your Mac HD directory and THEN unzip it, because you WON'T be able to see it.
Reboot and try running the installer again.
If it gives you any troubles, that's where the news gets bad. You'll have to reinstall OS 10.