[eluser]wiredesignz[/eluser]
You could create a user object variable in your model and pass that to db->insert($this->user)
EDIT:
Sorry it appears you are doing this already, I dont see the problem there should not be a cross over of data unless there is also a user object class variable also in your controller.
Using php5 you can make the model user class variable protected. This may help. Or you may need to rename the user object in the model.