You are supposed to be able to install on top of an old version. If that doesn't work, it's a bug.
The only thing I can think of that would cause this problem is that MB was already running. Maybe not in a visible state, but hung and only visible in the Task Manager.
I guess I'll have to do some testing on XP. I didn't bother to do that for this release, as it's never been a problem before
By the way, did anyone try it in Wine? Someone reported that it didn't work there anymore.