Rendered Animation Files won't Play

When I generate an animation in Ambient Occlusion Ex, the resulting MPEG files won’t play on my computer. Instead I’m getting an error message stating that “This file isn’t playable. This might be because the file type is unsupported, the file extension is incorrect, or the file is corrupt.”

If I try to open it in Windows Media Player, I simply get a message stating that “Windows Media Player encountered a problem while playing the file.”

Is anyone else having this issue?

Windows Media Player probably doesn’t have the proper codec. You can use VLC: