Go to Control Panel - Classic view - Folder Options.
Click on the File types tab.
Click on the New button
Click on the Advanced button.
Change Associated file type to the default for .dll -
Type in .DLL
click on OK
Click on Close
The default file association for a .dll file in Vista is Unknown application.
A Utility to Unassociate File Types in Windows 7 and Vista
Use Remove file association for .dll
That will set it back to Unknown application.
In Vista and Windows 7, it's easy to change the program that opens a file type by default from one program to another, but a puzzle to figure out how to change it back to the operating system's default in some circumstances.
That was easy in XP, back to Win 95.
In Vista and Windows 7, to change the program that opens a file type by default from one program to another....
Help and Support
Search for: file association
Click on: Change which program Windows uses by default