Strange Notebook Mouse problem
 

I was sent a file by email, and downloaded it into a folder. Ever since, then, every time I attempt to left-click, all icons/files to the left and above it become highlighted. For example, if I have a folder open, and I click on the icon in the bottom right, every single icon will be selected. It's a big problem, because now I am forced to navigate with right-clicks. Could this be a virus? Or if not, what kind of problem could it be?