Hi zedyll,
It's very suspecious that you can play the sample file fine the first time round but next time you get a "format error".
Generally the "format errors" are associated with either and incorrect video conversion or a HACKED PLAYER. If you say you can play the sample video fine the first time round, then it sounds like a HACKED player.
Try FORMATTING your player in Windows, otherwise follow this guide here -->
How To: Removing Memory Hack.