Codecs not accessible to all XP users

Not all users on my XP machine appear to have access to all installed video and audio codecs. Originally I noticed this when, after installing the DivX package, a "power user" started receiving error C00D10D1, claiming that the required codec is not available. Video was played, but not using the DivX codec. In the mean time, the administrator had no problems using DivX. The following codecs were being used:

Administrator - MPEG Layer-3 Decoder, DivX Decoder Filter
Power user - AC3Filter, "-"

After uninstalling DivX, the administrator was still able to play video, but the power user was not. At this time Administrator was using MPEG Layer-3 Decoder, "-".

At this point I re-registered qdv.dll using Regsvr32 (a solution suggested in another thread). After this power user was also able to play video without getting errors, but it still used AC3Filter instead of MPEG Layer-3 Decoder.

Once I re-installed DivX, the original problem resumed. Re-registering qdv.dll, quartz.dll, and a bunch of other DLLs didn't fix it.

It seems to me that the power user, another test user I created, and probably all users not in the administrators group simply can't access some of the installed codecs, though they all can access AC3Filter. However, I have no clue how to resolve the problem. Any ideas?