BUG #11731: installer reliance on default file association for vbs scripts

The following bug has been logged on the website:

Bug reference:      11731
Logged by:          Forg
Email address:      forgottenandlost@gmail.com
PostgreSQL version: 9.3.1
Operating system:   Windows 7 x64
Description:

The current windows installer expect the file declared file type of .vbs
fails to be "VBSFile". If the user installs an application that changes that
registry entry, the installation immediately fails claiming it could not
wright to a temp environment variable.

To reproduce this easily, run notepad++ and in its options set it to handle
vbs files, then try to instal PostgreSQL.