Run the DirectX installation program from the 9.0B folder (DX9B).

Note that copying NON-CAB files from DX9B into DX9C might NOT work (It didn’t work for me so I can’t guarantee it). Choose 'YES' when prompted to overwrite files. Copy all the CAB files from your DirectX 9.0c folder (DX9C) into your DirectX 9.0b folder (DX9B). Extract them on any folder you want, for example: DX9B (for DirectX 9.0b) and DX9C (for DirectX 9.0c).

You will also need 9.0c distribution package, so download it if you don’t have it yet.

Download DirectX 9.0b distribution package.

Instead, you’ll get the native DirectX 8.1 that comes with the original Windows XP installation.

Once your computer restarted, your DirectX 9.0b (or 9.0a) is gone.

I think all, if not most of them, will ask you to restart the computer. Run the program you’ve just downloaded.Most of them are freeware so don’t worry about wasting extra money. Search using 'directx uninstall download' keyword will do. So anyway, here’s the solution for the DirectX 9.0c installation problem. If it is '9” (including 9.0a and 9.0b), the program will think that you already have DirectX 9.0c installed since most Windows XP SP2 installation already come with that DirectX version.

If it is Windows XP with SP2 installed, it will then check the DirectX version, but only the major version (the number behind the dot). Technically, the install program (setup.exe) will check the Windows version first. This is a (stupid) bug from Microsoft’s DirectX 9.0c installation program. When the installation begins, it goes straight into the 'Installation Finish' dialog box. When your try to install DirectX 9.0c over DirectX 9.0b in Windows XP SP2 OS it won’t allow you to do so.

Directx 9.0 c installed