Computer freezes immediately after login

I am trying to fix my daughter's PC which runs Vista SP2. The computer
boots up fine but when you click on a user (either an administrator or
limited user) you get to the welcome screen and then nothing happens.
Pressing ctrl alt delete twice gives you the task manager and then it is
possible to cancel and then everything works ok after that.

I have run a virus check and also for spyware.

Please can you suggest a possible cause or what I need to look for.

Thanks