Password expired - change pw -> Access is denied

I booted up my Vista Ultimate this AM to a "Your password has expired and
must be changed message."

It offers me 3 fields, 1st one is filled in (my old pw presumably), 2nd says
"new pw", 3rd says "confirm pw".

I fill in 2, and 3, and click -> which results in a brief flash of
"Changing password..." then "Access is denied."

I've tried clearing 1 and putting in my pw instead of letting it get filled
in, the result is the same.

How do i fix this?

Thanks.