Troubleshoot codecs

Given this fiddly installation system, it’s annoying that Windows doesn’t include a tool for managing them. This means finding out which codecs are installed on your computer, let alone working out which ones are causing problems, can be difficult.

A good first step for any codec problem is to install a codec analysis program. One of the best is the free tool Installed Codec.

Run Installed Codec and (after confirming the UAC prompt if you’re running Vista) you’ll see a list of all the codecs listed in the Windows Registry. Most usefully, the program checks to see whether each one is physically present and will mark any that are missing with a red highlight.

If a codec is marked as missing, right-click its entry and select Properties. This displays all the information recorded in its Registry entry, including a full name and filename ­ putting either or both into Google will often be all you need to find and re-download the codec in question.

Another useful tool built into Installed Codec is the ability to list codecs in the order they were installed. This is particularly handy if you have media files that used to play correctly, but now fail to open. If this is the case, click on the ‘Installed/Modified time’ column to sort by date, and check to see if any codecs have been installed recently. If a new codec has been added recently, try right-clicking it and choosing Disable Selected Items from the menu before trying the file again.

Removing individual codecs manually isn’t very easy, but then it’s rarely necessary. Most codecs used by Windows are known as Direct Show Filters, and will be identified as such by Installed Codec. To remove one, use Installed Codec to find its filename and location (such as c:\path\filter.ax), then use the regsvr32 tool to de-register it as follows: open a command prompt, making sure to elevate to Administrator rights if running Vista, then type
regsvr32 /u “[drive:]\[path]\[filtername.extension]”
replacing the terms in square brackets as appropriate for the codec in question.

Add it up
If all the computer’s codecs are present and correct, but you still find that some media files won’t play, then normally it’s an indication you may need to add a new codec. This can, in some circumstances, be simple enough: play a common video file in Windows Media Player and it should be able to prompt you to download and install the necessary files. In other cases, though, there’s often very little clue given as to what kind of video or sound is contained within a file, let alone what codec is needed to play it.