Selecting a file usually opens it

For the last couple of weeks, if I try to select a file with a single mouse click, nine times out of ten it acts as a double click and opens it. I have already set all mouse speed options in System Preferences to lowest setting, but that seems to have no effect. This is making it virtually impossible to arrange files (I am a photographer and often have to shift around several dozen files).

Any help would be greatly appreciated, since as far as I know this makes no sense...