jon added a new FAQ about Problems installing Opus today. I've had some problems when installing the first and and second Beta on top of my installation where the installer couldn't replace some files so I had to uninstall/reinstall Opus.
Since I'm installing a new Beta every few days at the moment 
I'm using a simple trick to avoid such problems.
I close dopus.exe and dopusrt.exe using the taskmanager and afterwards I rename the folder "C:\Program Files\GPSoftware\Directory Opus" using Explorer. Now the installation runs fast and easy because there's no need to replace anything.
Don't forget to delete the old renamed folder after restarting the system.
What surprises me is that I have never been able to install Opus without a restart. Shutting down all obvious programs and processes does not change this.
I guess it has to replace something that is in use, but Opus is just about the only software I use that will not install without a restart.
Anything that install a shell extension requires a restart to update, since there is no way to force its DLL(s) to be unloaded from all the other programs.
For example, any program which shows a File-Open dialog will have dopuslib.dll loaded, and there's no way to make it unload it except restarting the program or the computer. (Well, the login session. Technically, the computer itself doesn't need to be restarted, except that the copy-on-reboot mechanism in Windows is tied to reboot and not login.)
Same reason (or one of the reasons, at least) Adobe Reader and TortoiseSVN require restarts (for example).
There is a restart manager in newer versions of Windows that allows installers to ask apps to restart without requiring a reboot, but it only works with the handful of apps that opt-in to that mechanism, so it's pretty much useless for stuff like shell extensions which are loaded into almost everything.