1] I should remove the old and add the new mouse when the power is off, right?
Yes, and No!
The OS doesn't really care whether two mouses are connected. But to disconnect the PS/2 mouse the answer is Yes. Unlike USB, anything to be connected/disconnected to a PS/2 port should only be done when the computer is shutdown.
But, connect the USB mouse before shutting down otherwise you may not be able to navigate around Startup without a mouse. It can be done using the keyboard, but not many use that nowadays.
2] Depends upon the mouse. If you connect the USB mouse before shutting down to disconnect the PS/2 mouse, then the question doesn't arise. In any case, you can still use TAB and Enter to select icons in the Welcome Screen. Test it if you wish whilst you still have the PS/2 mouse connected.
3] Windows should recognise the new hardware when you first connect it. I am not sure, but there may be some USB mice that come with a CD with software. If yours doesn't, then just plug it in and Windows will recognise it.
I doubt that will be an issue.
Hope that helps.
Keep that PS/2 mouse handy. USB mouses do fail and sometimes USB ports fail, so having a PS/2 mouse around is always very useful.
On an XP computer, I might have to switch from the PS2 mouse to a new USB mouse.
(1) I should remove the old and add the new mouse when the power is off, right?
(2) This computer has two accounts. You have to select an account at
startup, using the mouse. The first time I startup with the new mouse,
will it be activated in time, so that I can select an account?
(3) The first time you use a new USB mouse (and it's plug and play),
does Windows run some little program because there's a new mouse, or
does the mouse simply work right away?
(4) If the computer is old (late 2005, XP) and the mouse is really new,
does that increase the chance there will be a compatibility problem?