Do you have a small application called "FIWI" installed?, it creates a second instance of Finder to use for file managing, try to relog from Mac menu.
If not, its most like a corrupted LaunchServices database. Launch the Terminal app in /Utilities, copy & paste in this one-liner, hit the return key, quit the Terminal app, and restart:
/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchSe rvices.framework/Versions/A/Support/lsregister -kill -r -domain local -domain system -domain user
Ensure that there's no spaces in the LaunchServices entry.