Afaik, twitvid isn't actually a specific format... Firstly, try changing the file extension to something generic, such as .avi and open it up in VLC Player
If this doesn't work, try using gspot to analyse which codec is required to play the file - here's a quick start guide. It's free and very easy to use.
If none of that works, hit me up with a link to whatever it is you are trying to play and I'll see if I can convert it for you!