Well, I looked at the CPU usage in Windows System Monitor, and that says 100% too!
So it's only Process Explorer which is saying that it's only 4-5%.
Cool Beans System Info and Sysmon both say 100%.
Maybe PE uses different criteria to measure it?
Incidentally, WinTop also says that "Idle" is using nearly all of the CPU, as does Process Explorer.
This "problem" (if it really is one) does disappear if I uninstall Tihiy's thumbnail viewer.
I didn't actually delete any files, just unregistered gdipthms.dll, and ran "regsvr32 /i browseui.dll".
Obviously I lost the video thumbnails and preview frames, but the image file thumbnails and preview frames reverted to the Windows default, and the 100% CPU indication went away.
I'm pretty sure that it's the generation of thumbnails and preview frames from video files that's mainly causing the problem.
The results with video files seem to vary a lot too.
Some do nothing when highlighted, some produce a "Generating Preview..." message which then goes away but no preview frame appears, some produce a preview frame after a delay, some produce one instantly.
This is all from QuickTime movie files, and I can't see why some are being treated differently to others.
They vary a huge amount in file size, but are mainly all the same codec, Sorensen 3.
The results on other video formats seem to be similar, but the majority of my video archive is QuickTime files.
Hope this helps to investigate things.