Errors concerning a missing d3dx9_26.dll usually come up in connection with games and have to do with DirectX. If you are facing such an error, there are two steps you can try out to fix the issue. Before we get to these however, be warned:
Do not download any dll files separately from websites offering dll downloads. It is not safe to do this! Here are the steps you can try out to make things work:
1. Install the version of DirectX that comes with the game you try to play. In most cases, some version of DirectX is included on the game disk. Usually these are versions that have been tested for compatibility with the game they come with and contain everything you need.
2. If there is no DirectX installer included on the game disk or the error comes up in a different context, you can just try to update DirectX from Microsoft's website. This way you can be sure you are downloading safe content that usually gives you a complete package of the dlls you need.