I had this problem before, but now it's very specific.
If I drag an effect or transition (audio or video) into a sequence, the user interface locks up. The timeline won't move, the effects window won't open options (click on the triangle doesn't do anything), highlighting clips doesn't highlight.
I can play the timeline, or scrub, but just clicking on it doesn't move the marker.
Now, to get it responsive again, all I do is 'reset workspace' (which someone suggested against 7.0). Of course this resizes all my windows, but the system works again until I drag in a new effect/transition.
Windows 7, 64 bit, Professional
Nvidia GTX 560ti Card, and the problem is the same with Mercury Hardware or Software