I've noticed that with each install of DO9 on my Vista Business machine I get a notification from Vista that the program may not have installed properly and asks if:
I want to let Vista make compatibility mode changes.
The progam installed correctly.
I've installed every 900x version and I've always chosen that the program has installed correctly. I have never seen any problems because of this. Although these issues are included on list that occasionally pops up of things that Vista offers to check with MSFT for new fixes.
My Vista is running with UAC on, basically no OS customizations or modifications of any kind. All installed app's required a statement of Vista compatibility before installing. So a pretty "clean system".
It happens about 50% of the time for me. You did the right thing! If you click the other option then Vista will reinstall the program using XP compatibility settings which will mess things up.
Vistas detection of when installers may not have worked is somewhat rubbish and it doesn't do a very good job of explaining what the two options mean. I quite like Vista overall but this is one thing that could do with improvement (or ditching completely).
So far I've seen the warning with several programs' installers but not once has there been an actual problem that needed correcting.
It's an artifact of Vista. When you install an update, the system will ask you to reboot. If you do you won't get the dialog. If you don't then you have effectively terminated the install script with a non-complete code (well, I think this is what happens!) so about a minute later Vista will play dumb and ask you if you really know what you were doing after all:)
It's a generic issue with older installers and the new Vista procedures.
I've never seen it with Opus although Leo has reported it. Maybe this is because I reboot after the update install.