Shalebing

This is most likely not a problem with ImgBurn as such but lately, for some unknown reason, when I insert a disk into my DVD drive, ImgBurn automatically loads which I do not want. Previously it was either Windows Media Centre or Power DVD that was default (sorry, can't recall which exactly). I'm running Windows 7 and the problem occurs whether it is a CD or DVD (blank or recorded) that is loaded in the drive. How can I change the default from ImgBurn back to what it was?