Problem converting two .avi files into one

it seems to work fine until i try to play it in windows media player. the original two videos played fine. under output i chose h.264 and under encoding format: AVI encoding: H.264. i also tried xvid. i tried following this tutorial: http://www.youtube.com/watch?v=yRwrEU6KDYY , not sure what im doing differently. help, please

Windows Media Player can be a terrible choice for playing many kinds of movies because it has limited codec choices by default, and isn’t terribly intelligent about letting you know it’s missing a codec. I can’t play any H.264 files in WMP either, and XviD files depend on whether you have XviD installed on your system.

A very capable Open Source player that’s friendly with a broad range of formats and codecs is VLC from VideoLAN. Google that up and snag it, it’s very useful.

By “converting” you mean rendering it using the settings mentioned in your post or playing it back?