ARMA 2 not launching - very strange

I know is another topic like this, but this is slightly different:

I recently had Arma II installed on my PC, and I was running Vista x64. I upgraded my system and was having problems since I didn't format, but Arma II still ran fine. So I decided to format, but I couldn't find my Vista disk, but had my XP OEM one, so figured screw it, I'll just go back to XP until 7 comes out.

Well, I formatted, installed all drivers, installed Arma II through Steam. Yes, I have a Steam copy because nobody carried the game retail, and Steam support sucks for Arma. BIG regret buying it through them, but what can ya do. Well, whenever I click on the Arma 2 logo, it doesn't do anything. I just get the generic "preparing to launch Arma 2" and it closes. Thats it. No checkmark boxes, nothing.

I'm rather frustrated because Arma 2 is really the only fun game I've been playing recently, and the fact that it worked before on Vista, and not XP, strikes me as very odd. Especially with the exact same hardware. Specs are listed as follows,

Windows XP

Core i7 920

Geforce 260 GTX x2 in SLI

3gb triple channel DDR3 (2.50 recognized)

EVGA x58 SLI LE mobo

Try going to (steam -> my games) right click on arma 2 then choose (properties -> local files(or something like that) -> verify integrity of local game cache)

Fixed

Dwarden on Steam forums figured it out. DirectX needed to be updated with the March 09 drivers. Then it was the "bad serial" error but the FAQ helped me with that one.

Now to kick some 'daki ass with settings maxed out on XP.