Ask anyone who edits video for a living one of their biggest annoyances, and near the top of the list will be "too many video formats." MPEG-2, MPEG-4, WebM, MKV, FLV, Ogg, AVI, WMV, the list goes on. That doesn't even take into account the mixing and matching you can do with coding formats versus containers.
It's not surprising that a lot of video players, including Window's built-in Windows Media Player, don't handle every format. You might rip your home DVDs in one format and then find out you can't play it back. What can you do?
Fortunately, there are several open-source media players that do handle every format. The big name in the field is VideoLan's VLC, which I've featured as a download on my site for years. If you use it, however, you're going to need to be careful for a while.
A Turkish hacker let everyone know that there are two big flaws in VLC's core code. This code actually shows up in a lot of open-source media players, like MPlayer, so they're vulnerable too.