This is N900's video playback specification, directly copy and paste from Nokia's webpage:
Video sharing and playback

* Video playback file formats: .mp4, .avi, .wmv, .3gp; codecs: H.264, MPEG-4, Xvid, WMV, H.263
* Video streaming: H.264, MPEG-4, Xvid, WMV, H.263 in .avi, .mp4, .wmv, .asf and .3gp containers
* Video playback in landscape format
* TV out (PAL and NTSC) with Nokia Video Connectivity Cable (CA-75U)
* UPnP via WLAN supported


Obviously that is a bloody lie. I can't seem to play my video files at all. I keep getting the following message:
Unable to play media.
Media format not supported.
I've tried two videos so far, both are avi. Anyway, here's a screen of my video file detail, too lazy to type it up seeing how I can't copy and paste.
http://img339.imageshack.us/i/bleachd.png/
http://img339.imageshack.us/i/maouimage.png/

The one with xvid codec works half the time, the other half it doesn't. When it does work, it lags like crazy, the video that is. Now, the one that uses the H.264, it doesn't work at all. Both files actually get the same error message.

Also, I've downloaded mplayer, I can't seem to find the icon for it anywhere. Anyone know how I can access it? And, is there anyway to add more codec to the N900? Or at least where I can find a player that can play both my files and MKV files?