How do I fix a computer that won't recognise vowels?

Suddenly, some time ago, our desktop stopped accepting vowels in the password log-in. I suspect this is something a stepson has inadvertently done in settings (he has also switched the fn function on his user account on my netbook). The stepkids have accounts on this desktop without passwords, so we could get into the PC, but I still need to know how to fix the problem once I'm in one of their accounts, as we wouldn't be able to password into either of the admin accounts until it's sorted. We've tried various keyboards with the same results, so we know it's a PC problem.