When you attempt to install the MS04-016, WindowsXP-KB839643-x86-ENU.exe, security update on Windows XP, you receive:

KB839643 Setup Error
Setup cannot continue because one or more pre-requisites required to install KB839643 failed. For More details check the Log File C:\WINDOWS\KB839643.log

When you inspect the %SystemRoot%\KB839643.log file, it contains:

SOFTWARE\Microsoft\DirectX\Version is Not Equal To Specified Value

This behavior will occur if you upgraded to DirectX 9.0 prior to installing MS04-016.

NOTE: Windows XP is delivered with DirectX 8.1.

To determine of DirectX 9.0x is installed, press Start / Run / Dxdiag / OK. The DirectX version is listed on the System tab.

If If DirectX 9.0, DirectX 9.0a, or DirectX 9.0b is installed, download and install DirectX90-KB839643-x86-ENU.EXE.