Modern music and movie files can be very complicated due to a wide variety of software and hardware standards. There are often several ways to skin each cat, for example: converting a DivX movie to SVCD can be done with a number of different tools with varying quality. I guess what I'm saying here is don't panic if you can't figure it all out.

Fortunately, there are some sites on the net with comprehensive guides to doing this sort of thing. For software links for almost all kinds of converters, editors, joiners, etc., try this section of www.divx-digest.com. You can also review the many guides and articles on that site. Another very comprehensive site for video/film/audio editing and manipulation is www.doom9.org; they have an extensive collection of FAQs, guides, and software listings. Be sure to also visit their forums for access to some very knowledgeable folks.

Finally, DVDRHelp.com is another site worth checking for anything relating to DVDs, movie rips, encoding/decoding, etc. Be sure to check the forums for the most up-to-date information.

• GSpot - Start Here! Open your file with this program and it will tell you which codecs/filters you need and if you have them on your system or not.
• XviD codec - Koepi's Version and Nic's version - This is a very popular format these days, and is used often. Modern builds can also play back that various DivX and m4v (mpeg-4 video) formats, so it's a good one to install first.
• ffdshow - Project home (OLD) - Download latest versions here - ffdshow is another multi-purpose codec. With this one program you can play back a large array of formats: XviD, DivX versions 3, 4, and 5, m4v, 3ivX, MP43, MP42, MP41 (Microsoft's bastardized mpeg-4 formats), WMV7, WMV8, H263, H264, MJPEG, DV, Huffyuv, RV10, RV20, and others. It also has many advanced post-processing options. Because of the complexity of this codec, I recommend it for advanced users. Try the XviD codec above, as it has similar multi-format capability with an easier to use interface.
• ac3filter - If you have video but no audio this is likely what you need. You also need this if Gspot tells you that you have a file with audio format "0x2000" or 8192. If the sound is too soft, go into its Configuration panel and increase the Master Gain, and/or enable DRC ("Dynamic Range Compression".)
• DivX - This is the official site of the DivX codec, which is currently at version 5. There are three main "flavors" of DivX: the old DIV3 (which was simply a hacked Microsoft mpeg-4 codec), DIV4 (which was coded by Project Mayo and later became the XviD codec) and DIV5, which is the commericialzed product from the DivX Networks company. This codec is not free and often inserts an unwanted "DivX" logo at the beginning of playback. If you absolutely can't get a DIV5 file to play back then you may need this codec, however these days the open-source XviD codec (above) or ffdshow do a very good job at most DivX formats, so there's really no need to indulge a company that makes an adware-infested product.
• OggDS - This is a DirectShow filter for Ogg media files. Install this if you have an .ogm file. For more info on Ogg media, read this FAQ.

The codec is what does the actual encoding and decoding of the data -- hence its name. A lot of people ask questions such as "I've tried player X, Y, and Z but none work, which player do I need?" That is the wrong way to go about solving the problem as all media players use the same codecs (with some exceptions, below.) Once you have the proper codecs, any media player should work. Likewise, without the required codecs, no media player will work. Do not expect MS Media Player's automatic codec download to do anything. It's designed to only know about Microsoft codecs, which are hardly ever used for movies and TV episodes that you'll encounter with BitTorrent.

There are also some applications that have their entire own set of bundled codecs, which means they play most anything out-of-the box. VLC (Videolan Client) is the best known example of such a player. If you are frustrated with codecs, try installing VLC. It should be able to open just about anything you throw at it. An up and coming alternative is mplayer which was originally developed to play media files under Linux and non-windows operating systems. However, it has been ported to Windows, so you can use it in a way similar to VLC. However, it is currently hard to use, so try downloading a GUI front end to go with it. At this point mplayer is still for experts only.

It's very tempting to try one of these "all in one" files that claims to have every codec you'll ever need. That may be correct, but it's also the problem with them. They typically install a large number of things for the sake of being complete, but this is often counterproductive. I know that it seems like it should be the best idea but it's not. If you find that you're in "codec" hell then uninstall every last codec that you can find, and start from the beginning. With a very small number of codecs (for example: XviD, ac3filter, and perhaps ffdshow or the DivX codec) you should be able to play back the vast majority of files that you encounter.