Further information.
PC is Win7 professional 64bit. My app is 32bit.
I have changed the setup script to add "Register DLLs Immediately" after each of the two DLLs of mine (instead of at the end "Register all pending...") I think this now makes it work OK.
Also: I have the DLLs, OCX, etc set to only install if version/date is older than existing. I try installing it several times (and restarting PC after). But still, EVERY TIME I get the message after completion that PC must be restarted to complete the install. Why?
I have tried turning UAC off. Turning all anti-virus/ security software off. No difference.
These problems only seem to apply to Win7 Pro 64bit.