The problem is caused by QTKitServer and quicklood processes trying to create thumbnail/preview images based on partial or corrupt avi files. I get this when downloading an avi… the processes hog 100+% of CPU until the files are completely downloaded.
To fix: start Terminal and enter the following commands:
killall STOP quicklookd
killall STOP QTKitServer
This will permanently (until reboot) kill all preview/thumbnail processing – which fixes the CPU issue but will mean you have no preview/thumbnails for any files seen in Finder (a small price to pay for me!)