Every wonder why you are constantly asked to install DirectX 9.0c even though you already have it installed? Here's the answer as the folks over at Coding Horror have found an interesting
little trick that Microsoft has been doing with DirectX 9.0c versions. You'd think that since they haven't released 9.0d or 9.0e that it's been the same version for the last year or so right? Turns out that Microsoft has released almost seven versions since the initial release last year and while each version is completely backwards compatible they just didn't update the numbers (although it looks like Vista ships with DirectX 9.0L).