The K-Lite Codec Tweak Tool is a portable utility that can detect and fix a lengthy list of PC video problems.
By default it will detect and remove (with your permission) broken codecs, small programs that are used for encoding or decoding audio or video streams. If your PC used to play a particular video file type, but doesn't any more, then this may help.
The program can also help you reset default codec settings, or configure them in more detail. Having problems with Xvid videos, say? Check "DirectShow filter configuration", click Next, and, if it's present, click the Xvid Decoder button. Here you can set values like Brightness or Aspect Ratio that will affect how all Xvid videos will be rendered in future.
You're able to apply similar configuration tweaks for other filters. It's easy to list all your installed codecs and filters, which could be useful if, say, a video plays on one of your PCs but not another (run the tweak tool on both, look at the installed filters and note any differences). And you can also back up your codec and filter settings for easy restoration later.
And the Tweak Tool can even access some little known Windows Explorer settings. Would you like Explorer to generate thumbnails for all the videos you can play, for instance? Check "Thumbnail settings for Windows Explorer", select the appropriate file changes and you should be able to see the change right away.