The FP2 Server installer creates a user called PostgreSQL automatically. In order for the Server installer to run correctly, this PostgreSQL user requires read/write access to the \Program Files\PostgreSQL folder.
On most Servers, this new user will only be assigned to a default security groups which may not have the correct permissions to complete the installation. In order to ensure that this is not a problem, check the access permissions for the default User Group on the Server. If the Group has edit permissions set for the /Program Files folder, then there should be no problem.
If the default User Group does not have these permissions, manually create a /PostgreSQL folder in the Program Files folder and give the default User Group edit permissions on that folder. Then run the Server installer normally.