Nope I have Win XP (index off). Like I said to begin with MC is the only application that is running with sparatic CPU spikes..
USB drive is always on and plugged in with the PC.
I've definitely seen similar results on some of my single-core machines, when building thumbnails for XviD and x264 videos particularly. I know it has something to do with how MC calls the DirectShow filters to generate the thumbnail.
I can say that keeping the thumbnails well-built manually helps a lot. Also, a dual-core machine (with plenty of RAM) makes the problem completely irrelevant because MC is so well multi-threaded. With a dual-core machine, even though the CPU usage is still high on a single core (FFDSHOW's fault), MC's UI still feels snappy and responsive.
Though... As I've said again and again in these threads (which seem to pop up every so often). Some method to completely disable the on-the-fly thumbnailing, with an accompanying automation command to initiate the manual build, would completely solve this problem. Then you could just schedule the builds for 2am every Tuesday and be done with it.