You can change the program with which the extension is associated, under
START menu|CONTROL PANEL|DEFAULT PROGRAMS|Associate a file type or
protocol with a program.
Right click the file, select properties, on the General tab, there's a
section near the top called "Type of File," click the button the "Change"
button in that section.
You can change the file association using the "Default Programs" applet, but can't remove associations for a file type, using the GUI. This is one limitation in Windows Vista. Only option is to edit the registry.