Can't preview files in column view
Two things you can try :

If you don't need the preview feature in column mode (or for a short term fix), you can disable the preview column by going to View Options cmd-J in Finder and unchecking the "Show preview column" checkbox.

Alternatively, you can disable Quartz 2D Extreme (which should be disabled by default, but may be active and been known to cause Finder and preview slowdown). You can run the following command to disable it regardless.  

sudo defaults write /Library/Preferences/com.apple.windowserver Quartz2DExtremeEnabled -boolean NO


Run this command in Terminal (found in the Utility folder in Applications).  You will then need to reboot your Mac.