The movie was about a guy that lived with his mom who ended up getting bitten by some type of rat or something. Turned her into a monster and the guy had to eventually kill her. What is the name of the movie?
Braindead? It's made by peter jackson (lord of the rings), guy lives with his mom, and she gets bit by some foreign monkey thing and turns into a zombie creature and he has to kill aload of zombies and her on the roof and it's really gory.
http://www.youtube.com/watch?v=xbOmCUWm25w&feature=related
Braindead is AKA Dead Alive, and it's probably your movie.
No confirmation from O.P., but to my knowledge there is only one movie where a guy's mom is bitten by a Sumatran Rat Monkey and turned into a zombie, and indeed that is BRAIN DEAD/DEAD ALIVE. As Solved as they come.