Hi Mark,
I don't think you can make an application the default for a particular type of card, drive, etc. You can make it the default for a file type. I've only got XP in front of my now, my Vista is at home, but it should be as simple as right clicking on the file, going to Properties, and on the "General" tab there's a line for "Opens with". That tells you what the current default application is to open that type of file. There should be a button next to that labeled "Change". Clicking that should let you change what the default application is for that file type.
I'll double check these instructions on my Vista system at home tonight.
Hope this helps.