Unfortunately, I don't think your mouse will be a good instrument to practice on. If you are interested in the data from the mouse Ben's ideas are great, but I believe they just ask the OS what it is seeing, and does not actually look at the serial port.
The cause of the error, and the reason you can't use the mouse is as you expected. Windows only allows a single program to control a serial port. Even though you want to just read from it that is still considered controlling it, so windows is denying you access. You could use a mouse if you could attach a 2nd mouse to another COM port, and to then convince Windows to not think of it is a mouse. I am not sure if that is even possible. You may just be better off, hooking up your instrument and going for it. Another area to check is <a href="www.ni.com\idnet">Instrument Driver Network</a> which lists a lot of VIs for specific instruments. Your instrument may already have VIs programmed for it. Evan National Instuments
