hi, not sure whats gone wrong but i recommend using xmousebuttoncontrol, to add the extra functionality of per app config. eg exclude firefox to retain it's middle click features or sketchup to orbit wih middle click etc.
Pretty essential partner for CD if you ask me.
GL with fixing your issue, have you tried downloading it again.
xmousbuttoncontrol uses the hotkey to initiate CD from the middleclick which should fix your problem.
don't know if xmousebuttoncontrol will work with windows7