Hello,
As I understood, you are trying to install Symantec Endpoint Protection SBE 12.0 on Windows Vista 32 bit.
If incase, you are trying to Install
Symantec Protection Center on Windows Vista 32 bit, then to let you know that Windows Vista (32-bit, 64-bit) is not officially supported. Check this Article: http://www.symantec.com/docs/TECH93309
However, when you are installing Symantec Endpoint Protection Small Business Edition Client, then create an Custom Package and specify the installation path for C drive and not D drive.
Again, I see that you have said that, "I've tried typical and custom with the same results."
That clearly makes us believe that there is something wrong with the system. Does the same thing happen with other software installation??
Could you try running the Symantec Support Tool on your machine and check what does it specify.
Symantec Support Tool could be downloaded from here.
Hope that helps!!