> I am using Eagle Freeware under Windows

Me too, with XP and 98

> The reason may also be my mouse because it has some drivers
> installed

There's a note in the Eagle manual (manual-eng.pdf) page 44

Mouse Keys

The middle and right mouse button have a special meaning for a number
of commands.You can use the middle mouse button only if the operating
system knows your mouse is a 3-button mouse, that is your mouse
must be installed this way

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