The following bug has been logged on the website:
Bug reference: 13662
Logged by: John Gunter
Email address: jgunter@wsi.com
PostgreSQL version: 9.3.9
Operating system: Windows
Description:
In the "Base Directory" registry entry the Slashes are FORWARD.
The PostGIS installer uses this value to populate the install directory and
PostGIS will NOT install with this path
C:/Program Files (x86)/Postgresql/9.3