Not to intentionally spark a debate on whether or not to use registry cleaners (don't
), possible it may have changed or deleted a Reg Key or value. It may have "optimized" by turning off the Fast User Switching service.
To Check go to: Start, Control Panel, Administrative Tools, Services, then scroll down to "Fast User Switching" right-click "Properties" and set to "Automatic" and make sure it is "Started" restart computer. Then see if problem is still occurring.
*edited my spelling*