standard user can't log on

> I'm the administrator for my son's Vista Home Premium machine. I had set him
> as standard user, with parental controls. Suddenly he can't log on (system
> hangs when he tries; CTRL-ALT-DEL brings up the "log off" option which gets
> us back to main log-on screen). I've tried rolling back to known good
> points, problem still happens. I've tried with parental controls on and off.
> I've tried creating new standard accounts -- same problem recurs. Any
> suggestions or thoughts? Thanks. All of a sudden my Vista Home Premium machine is doing the same thing, but it is also with administrator accounts and standard user accounts. The screen also goes blank. I tried setting up new accounts and doing the same. Would love back any feedback possible, have no idea how to fix this.