PPRuNe Forums - Vista Windows Media Player and *.wma files.

Until today, whenever I downloaded *.wma files they would open (and play) with RealPlayer. Sometimes I'd get a pop-up saying that Windows Media Player was unable to play this file - but it would try anyway (with a tick-box suggesting that it would never ask again). I considered this to churlish, as I'm sure that *.wma files were created to be played by WMP, and having the latest version installed I expected it to WORK.
I downloaded the *.wma Last Post sample from http://www.banquetmusic.co.uk/Last_Post_sample.WMA, and first time around it actually played (on Real Player) but I (foolishly) decided to 'fix' it so that *.wma files played on WMP instead of RP, so I delved deeply, 'show file extensions' and found a resident *.wma file and changed the option so that it was associated with WMP instead of RP and ticked the 'always' box.
So far so good, and clicking on the (stored) *.wma files did, indeed, invoke WMP.
Back to the Last Post.WMA (is this significant being upper case?). Up comes the pop-up referred-to above and I ticked the 'don't bother me again' box, assuming that things would be fine from now on.
Wrong!, now it won't play (Last Post) but comes-up with yet another pop-up:- http://i24.photobucket.com/albums/c1...n/304b3493.jpg and no further action occurs!
I'm using Firefox BTW if that's relevant.
Question - how do I UNDO the ticking of the box?
(the WebHelp opens http://www.microsoft.com/windows/win...nalid=8007000D which doesn't help me much - maybe I'm short on understanding . . . ).
Strangely (or not?) I accessed PPRuNe (and the Last Post website) using Internet Destroyer (don't ask) and the clip played OK in Real Player, but when I returned to Firefox (my 'default' browser) up comes the pop-up shown above and nothing is heard! - and there is no option to 'undo' AFAICT (thanks, Bill!).