i installed win64 and had all sorts of issues because of drivers...have you tried going to the control panel > mouse and looking under the buttons tab?

-if the one windows is intalling automatically for u is only making it a 2 button mouse, its might be the driver its intalling...try changing the driver. Goes to the mouse manufacturers site and try getting the exact driver for that model...but than they might not have one for win64...can always try another mouse too