Because DirectX 8.1 is considered "legacy" software, official direct downloads from Microsoft for this specific version are often redirected to newer runtimes. For original, untouched installers, you can use these archived sources: Microsoft DirectX 8.1 SDK full - Internet Archive
: DirectX 8.1 is already built into Windows XP. If you are running XP, you typically do not need to install it unless a specific game requires a manual re-installation of components. download directx 81 offline installer link
: These systems include newer versions (DirectX 11 or 12) by default. To play DirectX 8.1 games on modern OS, you should install the DirectX End-User Runtime rather than the original 2001 installer, as it adds the necessary legacy files without overwriting your modern system files. Key Features of DirectX 8.1 Because DirectX 8
: Download the Microsoft June 2010 Redistributable, extract it to a folder, and run DXSETUP.exe to install all missing legacy libraries. : These systems include newer versions (DirectX 11
Features. Refractions with the use of a du/dv map. Dynamic shadows. Directional lighting on world brushes using normal maps. Cube- Valve Developer Community DirectX End-User Runtimes (June 2010) - Microsoft