I have just installed FFdShow ver 18.104.22.1686 in my XP Pro SP2.
The installation proceeds fine in Administrator's account. Testing using Real Player 11 and Media Player 10 when running in Administrator account works fine and the FFDshow Audio and Video decoder tray icons are showing.
However, when I run the above players in LUA (Least-Privilege User Accounts) or as normal users, they fail to play the same media file and the FFDShow decoder icons are no where to be seen in the tray area.
I believe it is because FFD Show does not comply with the Windows Security Model and hence does not work when used as normal user.