Problem:

Although users with Super Users privileges have no problem to install Net Express 5.1, they are unable to apply any WrapPack and get the following error message:
"Windows Install error 1605"

Resolution:

Installing Net Express and its WrapPacks (or Updates) requires Administrators or Super Users privileges, but a corporation may put a system policy in place to prevent users from installing or applying software update.

Windows Install error 1605 actually means "This installation is forbidden by system policy"

In the system policy, there is an option to prevent non-administrator accounts from applying software updates. It is called Prohibit non-administrators from applying vendor-signed updates.

Since Super Users are non-administrators, they may be able to install software, but not to apply updates. The users will then need to request their IT group to disable the option temporarily or have someone with administrator's privileges to apply the WrapPack on their behalf.