There's a lot of confusion going on out there. Increasingly it's more and more about issues related to DirectX. So much so that now I'm writing an update to the article I wrote almost 3 years ago now. That article advised people to install the latest DirectX end-user runtime if they encountered errors like d3dx9.dll is missing or not installed.

The confusion seems to stem from people running Windows Vista or Windows 7 and seeing that they have DirectX 10 or 11 installed, by way of the DirectX Diagnostic Tool.

DirectX 10 however represented a clean break from DirectX 9, essentially the two are on the system together. DirectX 10 and up handles all the new stuff and DirectX 9 provides all the backwards compatibility with previous versions.

It can be represented like this:

The DirectX team felt this was superior than bloating out DirectX 10 with all the old features nessasary to support ten years worth of games. Why do that when DirectX 9 would be on the system anyway and can achieve all of that?

The most recent example I came across, about 30 minutes ago. Had a user attempting to play Flight Simulator X, which returned the following error:

Flight Simulator cannot run because the version of Microsoft DirectX(R) installed on your computer is incompatible. Please reinstall DirectX9.0 by running Flight Simulator Setup, or download current version from www.microsoft.com/directx.

Nice and simple, and tells you exactly what to do. Go to the DirectX website, and grab the latest version. Doing that would solve the problem.

However, as people think they have DirectX 10 or 11 installed, they don't think they need to grab the update. Not realising that DirectX 9 has its own updates which may be required or that it has optional components which don't come installed on the system.

My advice for those with problems, just install DirectX. It doesn't matter what version you have, grab the update anyway. It won't break anything by installing it, it'll know what optional components are needed and which components need to be updated and update them.

For those installing a game, and are tempted to cancel the DirectX installer, just think for a second that maybe that installer is installing something the game needs to work.

So this error message has been doing the rounds lately.

This application has failed to start because d3dx9_32.dll was not found.

Or other error messages around d3dx9.dll, dxdx9d.dll, d3dx10.dll, xinput.dll, xact.dll, d3dx10_33.dll, d3dx9_40.dll and other variations being missing, not being found or installed.

These are all optional components to DirectX that may not be installed with the version of DirectX you already have on your system.

Typically these will be installed when the game installs DirectX, however for historical reasons a lot of gamers, knowing they had the latest version of DirectX didn't bother (I never do either). Microsoft a couple of years back provided these optional components in the SDK, and as such may not be installed with the version of DirectX present on your system.

Games I've come across with this happening range from

Age of Empires III (3).
Battlefield 2.
Battlefield 2142.
BioShock (unconfirmed)?
Caesar IV (4).
Civilization IV (4).
The Elder Scrolls IV: Oblivion.
Ghost Recon Advanced Warfighter.
Ghost Recon Advanced Warfighter 2.
Hitman Blood Money.
The Lord of the Rings: The Battle for Middle-Earth
The Lord of the Rings: The Battle for Middle-Earth 2.
Lost Planet.
Need For Speed Most Wanted.
Virtual Skipper.

I'm sure there's probably a couple of dozen more games, feel free to list any additions in the comments below.

The solution is to either install the version of DirectX on the game's disc, or download the latest DirectX End-User Runtime from Microsoft (at the moment this is being redirected to the Games for Windows website), or from the Download Center.