Our installers are built using Setup Builder from Linder Software: http://www.lindersoftware.com/
Friedrich Linder has responded to our query as to why this might be occurring:
We hope that this resolves the issue for all of our clients.If this is on a UAC-aware operating system (Vista, Windows 7, etc.). then he has to run the "Clarion 6 UAC Installation Fix Version 1.30"
If he copied his Clarion environment over from another machine (means he did not install it) then he has to install C6 (Gold) and the 9055 patch to have a "valid" install.