iP4500 paper source changing randomly

I have set the driver of my iP4500 to disable the paper source setting of the application software (and I've set my default source as the cassette). Everything worked fine for months but now I'm finding that sometimes it selects the rear feed when I expect the cassette. When I go to the setting above, I find it's changed itself back to "enable..." So I duly tick the box and all's well for a while - then the problem arises again.

Has anyone else found this (and a solution)? The OS is WinXP SP3 and the Canon is the default printer (I have another printer - HP B9180 but that's not the default and it's ethernet connected).


john