The first item to check is make sure you are trying to install the proper bit version. I see you have x64 in your specs, so the version of PD12.5 should be x64, and not x32.
Check to see if the previous uninstaller left any remnants of PD12.5 in either the Program or Program (x86) Folders, such as an empty PD12.5 sub-folder.
Have you tried to install PD12.5 in Safe mode
Check to make sure your SYSTEM Properties have full control.
This KB Article is geared towards MSOffice, but the same rule applies: You receive an "Error 1303. The installer has insufficient privileges to access this directory" error message when you run Office 2000 Setup
See the Resolution Section.
In order to check, see: Permissions - Allow or Deny Users and Groups
Use Win Explorer to go to the drive you are checking, then see if SYSTEM has full control.
I have the feeling the previous version of PD12.5 did something to your permissions.