can’t log back in after screensaver lock

screensaver kicks in after period of non-use on xp box. user tries to log back in. login screen only half-way disappears, screen not refreshing. user waits a few minutes and ctrl+alt+screen comes back as if user never tried logging back in. userthen tries again and either can get in or can’t. if can’t get back in, this cycle continues. if a DIFFERENT user (admin) tries to log in, allows to kick user off, then user can log back in successfully. have tried disabling nic powerdown and thatseemed to help for a while, but problem comes back about a month later. tried disabling hybernation and servicepacking, same results. has happened on 4 machines so far.