this is usually a situation with the wordpress tables. since you say that you haven't installed or upgraded and I assume you haven't moved hosts or edited the table information, my first thought is that one of your database tables has gotten messed up a little bit.
do you have access to your database through mysql administrator, phpmyadmin or ssh?
if so, find your wp_options table and right click & hit repair. in ssh...I'm not 100% of the commands for that, but I'm sure google can show you a couple different ways.
as always, be sure to back up your db before mucking around in it.