How change file association?

I need to change program that opens xml files on my Windows Vista.

When I open "Open With" dialog and click "Browse" and locate my ___.exe, nothing happens.

The program is not added to the list. Vista just ignores my choice.

This exe file is on my D: partition in my DropBox folder. I wonder WTF?

assoc: Displays or modifies file name extension associations. Used without parameters, assoc displays a list of all the current file name extension associations.

and

ftype: Displays or modifies file types used in file name extension associations. Used without parameters, ftype displays the file types that have open command strings defined.