The only thing I can think of is a setting in InputDirector that centers the mouse when you switch to the other PC, though that's supposed to only happen when using hotkeys to switch and it is not a default option I believe. Any other software that's trapping the mouse? I'd start shutting down apps until it went away, then see if it starts doing it again. Also, did this only start happening after updating to ISBoxer 40? You might check with Lax and see if you can get an older version to test with if that's the case, just to rule out ISB as part of the problem.