Password incorrect on first boot

[ihaveaq]

So here's a new problem on my mum's PC that started yesterday. Upon switching the machine on from a complete shutdown (has been off for over 12 hours) it came up with the usual login screen and the "Password incorrect" message in red. BEFORE we touched the keyboard. We logged in, and as a security measure changed the password. Rebooted a few times, message hasn't returned.

Then switched it off last night again for 12 hours, turned on - same this now, "Password incorrect" even though we've just switched it on.

Any idea of what might be causing this?

Thanks