I'd say uninstall itunes but to disable the itunerhelper and ipodservice from running:

Right click my computer go to manage
Expand services and applications
Click services
Look at properties of offending service and set it to disabled.

As for keeping it from taking over did you look at the itunes perferences and see if it has someplace to set what file types it should be the default player for. I know quicktime and windows media player work like that.