I have the same issue, I tried starting regular plain old M&B, and found it no longer accessible, my key is a 5x5 key bought in 2008 I think, it now wants 4x4, but I can't convert it in the warband key converter thingamabob.
EDIT, I figured that buying it through steam was warranted, it still is only about 20 euro's for a game I spent so long playing