Have you tried doing the same thing in Explorer in case it isn't Opus?
If it only happens when using Opus then it's possible that the problem is caused by certain files or types of files on the DVD you're browsing either crashing an Opus plugin or a shell extension.
In that case, try following the troubleshooting steps in this FAQ:
Crash, exit or high CPU usage when viewing certain directories
It's definitely not normal for browsing a DVD, on Vista or otherwise, to cause Opus to stop responding. I've been using Opus on Vista for a year now without ever seeing that.