can’t get past login screen at login

user tries logging in after machine left on over night, name and password entered, user waits 1-3 minutes, never even gets past login screen, then gets kicked back out to ctrl+alt+del splash screen. tries again, same results, has to reboot machine before can login. have tried
rebooting before going home for the evening, and even disabling diskeeper, still no luck. Tried disabling hibernation and running sp1 again and hotfixes, worked great for 4 days then leaving it on over 3-day weekend caused same problem.