Try booting with your extensions off (shift at startup). If the problem goes away, its an extension.
Holding the space bar at startup will allow you to disable the mouse extension via the Extensions Manager; do this and see if the problem goes away.
I suggest deleting the mouse driver and using USB Overdrive - its better, IMO.
Before all that try trashing your Finder Preferences and System Preferences (a PRAM zap [Apple-Option-P-R at startup, let go after the third chime] woudln't hurt either).
You MAY (however unlikely) need to replace the System and Finder, but lets take things one step at a time...