I have had this problem for quite a long time now, and finally the aggravation has got the best of me. I have a unified logitec keyboard and mouse combo. In GRUB and in windows the keyboard and mouse work fine together. When I boot into xubuntu (12.04) they are not recognized. I have to pull the usb dongle in and out several time before finally both will be recognized. After they are recognized they wrok perfectly for the rest of the session. Any hints?