Alright, I'm running Ubuntu Hoary (stable) and I was using totem-xine to play my video files and everything was working fine, and all of a sudden, any file I play, except for .mpg/.mpeg, stops working. I get the error "Video codec 'XviD format' is not handled. You might need to install additional plugins to be able to play some types of movies" 'XviD format' changes depending on whatever I'm playing obviously.

I can play audio fine. As far as I know, I have all the correct codec packages installed, because everything was playing fine in totem, and these videos will play in other players (MPlayer), so I'm thinking I must have changed something to make totem stop recognizing that I have the required codecs, but I can't figure out for the life of me what it is. I've also tried uninstalling and reinstalling totem-xine and w32codecs. So, I'd love to get this working if anyone could kindly help me out, thanks in advance

Xine has two components: xine-lib xine-ui

The xine-ui component is just a graphical interface. Xine-lib however is a backend library that all Xine-based video players use. I order for xine-lib to support win32 codecs is needs to be built that way from source. Some distros build it with win32 support some don't. Have you made an general update or something lately? If you're unlucky you may have ended up with a Xine-lib that doesn't support win32 codecs.

Do you have xine-ui installed? If you do try typing xine in console to run Xine's own interface just to make sure this is not a problem with Totem.