If you search for DirectX here there is a link to an uninstaller someone made that might work in your situation. It makes the process seem more complicated than it is and doesn't necessarily do it cleanly due to the huge number of reg entries left behind. (Why did they make an installer and help file for their own simple inf?)
The universal DX uninstall method is: run dxdiag.exe and save the list of DX Files and Drivers. Make a batch file from this to delete them all from windows\system and windows\sysbckup. In Reg Editor del all Direct* keys from Local Machine\Software\Microsoft (may not be necessary but can't hurt) then reboot into DOS and run yer bat. Reboot into Win and run a registry fixer to del invalid entries. Ontrack Fix-It Utilities 2000 is highly recommended and much superior to Norton's.
If you check dxdiag or dx properties at this point you might find your system may have fallen back to previous files where possible like DX3 or whatever you had previously installed. You can delete those if you like from within Win (or DOS where not possible) but it isn't necessary.
Now you can run the DX installer of your choice, reboot and run the reg fixer again and then check for any problems in dxdiag.exe. Works for me
If anyone wants to try my DX8 remover files I can send them but I would not count on success without a good reg fixer!