That's a bit strange; it sounds like your Launch Services database needs to be rebuilt. It's easy to do, but requires the Terminal: you can cut and paste the following line to accomplish the fix (reboot after):
Code:
/System/Library/Frameworks/ApplicationServices.framework/Frameworks/LaunchServices.framework/Support/lsregister -kill -r -domain local -domain system -domain user
Scary as it seems, that should take care of it!