worker_thread_returned_at_bad_IRQL

Hi,

My 2 month new laptop which is installed with Vista gives me this message with a blue screen occassionally if I leave it on for several hours with no activity. “worker_thread_returned_at_bad_IRQL”
and it doesn’t restart or anything, I have to manually turn it off and back on.
Does anyone know how I would go about fixing this? Or even what it means? I’ve ran norton antivirus and nothing comes up.

Thanks
Joanna