This is a tool to help manage & sometimes repair the codecs & Direct Show filters Windows & often your installed software uses when working with &/or playing video.
http://www.videohelp.com/tools/Codec-Tweak-Tool
As always, backup or at least set a Restore Point in Vista/win7 -- notes don't say whether or not Codec Tweak Tool works with win8 -- & ERUNT is a great option to back up the registry in XP where Restore Points don't put everything back as it was. Similar apps that might be useful include Win7DSFilterTweaker [ http://www.videohelp.com/tools/Preferred-Filter-Tweaker ] & Filmerit [ http://www.videohelp.com/tools/Filmerit ] -- there are a few others you can find at sites like videohelp.com. AS briefly as I can, here's why apps like Codec Tweak Tool exist...
When an app tries to open a video file, it may/may not read the file itself to determine what kind of video it actually contains, e.g. AVC/H.264, mpg2 etc. -- when an app won't open a video file, if it doesn't read the file 1st, you can often get away with just renaming the file, e.g. somefile.VOB -> somefile.MPG. The app's next step is to call or open the particular files that app uses to decode that video -- those files may be self-contained in or with the app, e.g. VLC, or it may look to see what you have installed already in Windows that can do the job. Limiting the software to just use those video decoders that come with the app is safest, & as with the video converters that are on GOTD from time to time, it works reasonably well, but it limits the good as well as the bad, since apps designed this way can't use or encode any formats that the developer(s) hasn't seen fit to include. Here's what happens when video software doesn't take that safe route...
Windows uses two methods for handling video files, the oldest being VFW [Video For Windows], which Microsoft has been unsuccessful killing off, though it has tried for years. VFW is the simpler of the 2 & it works -- *usually* when there's a problem it's because a codec is set to work with one video format & you want to use another codec for that video. One cure is to change the "CC" code for the video file, then set your preferred codec to work with all videos having that CC code. More often it's taken care of by setting your preferred codec as the default for whatever type of video, either using that codec's settings [if/when available] or by re-installing your preferred codec. Often VFW codec problems are caused when some video app includes a codec installation as part of its setup routine.
The 2nd, more modern method Windows uses to handle video files is Direct Show, though when I say more modern that's really relative to VFW, as DS has been around for years & years. When an app uses DS to open a video file it "builds a graph", which means it assembles a chain of separate DS files called *Filters*, each with its own special job to do [there's a Microsoft app called Graphedt.exe (along with a couple of re-worked graphedit apps like GraphStudio) that allow you to *build a graph*, assembling a chain of DS filters manually to do whatever job, experiment & learn, &/or troubleshoot]. At the very least you need one DS filter to open the file, another to decode it, & a 3rd to display it on screen, though often there will be several more doing things like splitting the audio & video into separate streams, adjusting the color space used & so on. Here's where things can turn Nasty...
DS filter "X" might not work with DS filter "Y", or it may not work at all [even though it tells Windows it can do a job it may lie], or it might not be compatible with whatever app you're using, or your video might be slightly *off* so filter "X" can't work with it as-is. And you can have all sorts of DS filters installed, so there are several possible filter chains or graphs that might be put together using various combinations of those filters. Windows &/or your video software tries to build a graph, & if that combination fails, it tries again, & if that one fails too, it tries again, & again & so on until it gives up. Which DS filters are used 1st depends on their *Merit*, a number stored in the registry that ranks the filters from Always use to Never touch this -- often increasing the merit for the DS filter(s) you want to use, decreasing it for those you don't want gets things working. HOWEVER, there's no guarantee things will work that way, and, once a DS filter gets opened or called, it may stay open to conflict with another DS filter(s), Even Though It Isn't Used. In some cases all you can do is remove the offending DS filter when/if possible, or break it when removing the filter can't be easily done.
The whole mess can get pretty hairy... think of herding cats. Lots of video-related apps install their own DS filters, & they may/may not register them with Windows so everything else tries to use those filters too, & the app that installed those filters may/may not work without those filters installed & working. And then there's the part where you have to track down just which DS filters are giving you problems, which is often not obvious at all. PC/laptop graphics hardware adds yet another wrinkle -- that hardware more than likely includes some sort of acceleration or assist so you can play HD video [e.g. ATI has included it since their 2xxx series, & Nvidia's included it for just as long]. Your graphics hardware may or may not be capable of Nvidia's CUDA, AMD/ATI's Stream, Intel's Quick Sync, or the Open CL all 3 of those use today, but it still has DXVA, Direct X Video Acceleration, which means Direct Show. If something's not working, e.g. a video won't play, it could just as easily be caused by a conflict between the DS filters the player wants to use & your graphics hardware driver set.
At any rate if you have these sorts of problems, which can cause poor video playback as well as no playback at all, the Codec Tweak Tool may help fix or diagnose the problem. Of course the best practice is to be very cautious when it comes to installing Anything that has to do with video, realizing that some apps are really asking for trouble while other software like VLC leaves Windows alone, untouched. [That's BTW why I cringe every time someone recommends Format Factory -- one of the most intrusive apps I've seen, it send roots deep into Windows that can choke other video apps.]