I started with SourceTree 1.3.2 installed and wanted to update to the latest version. To do so I selected Tools > Check for Updates to begin the "auto" update process. However, during the installation the installer displayed the following error message and would not let me continue with the installation (though I could cancel it.

---------------------------
SourceTree
---------------------------
The installer has insufficient privileges to access this directory: C:\ProgramData\Microsoft\Windows\Start Menu\Programs\Atlassian\SourceTree. The installation cannot continue. Log on as administrator or contact your system administrator.
---------------------------
Retry Cancel
---------------------------

Putty nor Pageant did not appear to be running, though an SSH-Agent.exe process was. Killing that process and re-running the installation did not resolve the issue. However, I was able to resolve this by logging out and then logging back in; I did not need a full reboot.