If your original mouse is just a standard mouse then no need to un-install. You can find reference to your mouse from control panel, then click on printers and other hardware, you'll see the mouse Icon then.
If you need to remove drivers for a specific mouse, then this is done from Control panel, add/remove programs.
It will be quite safe to go ahead and install the driver for your new mouse without removing the standard one, [PS2] in fact that will remain there all the time. j.