Go to /Library/Contextual Menu Items and see if there is anything (3rd party stuff) in there. If there is, try removing whatever is in there then check to see of the problem persists.
If that clears up the problem, then try adding back (one at a time, if applicable) any items you removed until you find the one that was causing the trouble. Get rid of that one.
I remember hearing that Stuffit installs a contextual menu item that's been problematic. If you installed Stuffit (the full version, not just the free expander which doesn't install a contextual menu item) then that might be a good place to start.
You can also check your home directory library and see of there is a contextual menu folder there. If there is, same drill as above.