IE7 is a core component of windows vista you cannot remove it. I do not see a reason to remove it as well. You can instead set firefox / opera as your default browser. Even if you manage to remove IE you will render the system unstable.
I suggest you to remain it as it is and install firefox or opera which you want and use it. BTW why you want to uninstall IE7 Is there any specific problem with it or just want to use another browser