You could pull on the mouse and see which wire moves. Or you could tie a small piece of string around the mouse wire, and follow it until you get to the back of the computer.
Horseflesh, thanks for saying I’m doing a good job of posting without a mouse. I try, sort of. 
Actually, I figured out what the white line was. Somehow, I managed to click the “Set as Wallpaper” option on a webpage, and it saved it as “Internet Explorer Wallpaper.” Weird…
However, I still think I need a new mouse, keyboard (maybe), and/or hard drive. (hey, if this one is running at slooooooow speeds, there must be something wrong with it!) Besides, my sound volume has crapped out on me as well. Sigh…
F_X