Soft Problem

Sometimes you get a video file, produce it or download it from somewhere, or even get it on a dvd or cd and you are unable to see it. Assuming the video is playable and there have been people who have checked that, there can be only one cause:
The codecs from your computer are not appropriate to play that specific video.
Here’s what you need to do:

For Beginners

What are codecs? Codecs are programs used to “encode” a movie so that it occupies little space on your hard drive. Since the movie is a succesion of images (usually 25 images/second) you can imagine that a two-hour movie would take several tens of GB. So, in order to deal with the space problem and to be able to put the movie on a dvd or cd we use “codecs” to encode the movie, and thus make it smaller. Now once you have encoded it, it’s ok you have the small file, but to play it you need to “decode” it first. This is where codecs come into your practical use. Players (such as Windows Media Player for example) get codecs from where they are installed. So you just have to install the codec and your player will know how to use it.
Unfortunately there are loads of codecs these days, so the movie may be encoded in several ways, and you cannot just search for a new codec every time you want to see a movie.

Anyone prefers a solution that has “All in One” and that is exactly what you can find here. If the previous link does not work, you can get the codec from one of the mirrors here.

Here is some of the extensions that “Codecs_Allin1.exe” can provide for: .avi, .mpg, .mpeg, .wmv, .vob, .divx, .xvid, .ogm.

For Advanced Users

Similar to Codecs_Allin1 (as presented for beginners) there is the K-Lite codec pack, which you can download here:
http://www.free-codecs.com/download_soft.php?d=5012&s=775
If the link above does not work you can download it from one of the mirrors here:
http://www.free-codecs.com/K_lite_codec_pack_download.htm

K-Lite has much more settings than Codecs_allin1 and there are not set automatically by default, which is why you might experience displeasure when you play a certain movie (video falls behind audio or is unclear). However, if you take your time to set things the way it is best for you and/or your computer, you will obtain a better performance.

Codecs_allin1 automatic settings keep it at bay from DVD video files (such as .mkv files) where most often are necessary specific settings to view it right. K-Lite is also excellent for those with a slow computer. Manual settings can be made to improve your experience, even if normally that format would not work right.

Here are some extensions that K-Lite provides for: .avi, .mpg, .mpeg, .wmv, .vob, .divx, .xvid, .ogm, .mkv.

Permanent solution

However, codecs might not provide for all of the video formats that exist, and also keeping loads of codec packs would eventually slow down your computer performances.
For this kind of thing, you can use “Avi Video Converter”, which you can find here:
http://softproblem.com/2008/11/20/avi-converter/
This software can convert any video format into basic “AVI” that most players are able to play with a minimum pack of codecs.