Via [WayBack] Anyone with a hint on how to work around this: … “Please re-run this installer as a normal user instead of”… – Jeroen Wiert Pluimers – Google+
This happened for instance when trying to install Source Tree 2.x on Windows (1.9.x works fine):


[Window Title]
SourceTreeSetup-2.3.1.0.exe
[Main Instruction]
Installation has failed
[Content]
Please re-run this installer as a normal user instead of “Run as Administrator”.
[Close]
The problem was by accident the machine got in a state to run commands without UAC approval, so the run dialog would already look have “This task will be created with administrative privileges”:

It was odd, as the machine didn’t have it enabled in the security policy (secpo.msc):

So I did a bit more digging, bumped into [WayBack] Why does my Run dialog say that tasks will created with administrative privileges? – The Old New Thing and had one of those #facepalm moments: Explorer had crashed, and I had started it from Process Explorer, forgetting Process Explorer had an UAC token.
The solution is easy:
- Logoff / Logon
- Verify the Windows-R shows a “normal” run:

Then you can just run the installer:

–jeroen