Not sure if this is a problem with the hardware or software, but it has only happened (to us) on 620 model machines. Our workstations are a mixture of Optiplex GX260, GX270, GX280, 620, 745, 755, with between a few hundred and a couple of thousand of each model. On about 1% of the machines where SP3 has been installed, the USB mouse stops working, and these are *all* 620 model workstations. We tried reimaging a couple of these workstations with our SP2 image, and the mouse works fine until SP3 is installed, at which point it stops working again. Only a small percentage of 620 workstations are affected, and no workstations of any other model are affected.