Change default "open with" back to NOTHING?



Vista Enterprise SP1

I clicked on a certain file (extension .frm) and got the dialog asking
whether I wanted to open using a web service or choose from a list of
programs. I chose the list of programs. I forgot to uncheck the
checkbox, so now Acrobat Reader is permanently(?) the default double-
click open-with program for this extension.

I know how to change it to any other program, but is it possible to
restore everything to its pristine state such that there is no default
open-with program for this extension? In other words, how can I
uncheck that checkbox and make it stick?

(I recall asking this years ago with Windows 98. Why is the checkbox
pre-checked??? It should be left unchecked by default. Stupid.)

Thanks.
.