'OPEN WITH' in PROPERTIES, disappeared...

Hello,

Long story short, I changed the 'open with' of my .001 file into shell32.dll (I went to system 32 and just typed it in).

I did this by right clicking the file, went to PROPERTIES, then at OPEN WITH, clicked CHANGE, then BROWSEd over to WINDOWS32 for shell32.dll by typing it in the file box and clicked enter.

Now I regret my actions and want to change it back to its default orginal (which was: Windows Shell Common DLL).

BUT

Now the menus and stuff on the files' Properties is totally different. And what's more, the Open with change button and all is gone!

How do I put it back to the way it was?