This should really be in the editors hideaway - however as its late i`ll let someone else moan about it
A lot of these occurences have happened when importing a new database. The only suggestion I would have is to unistall and then re-install the database. If is still happening after re-install try and contact the programmer of the imported database and see if it is a known bug.
Unfortunately this is quite common with imported databases. But do also have a look in the editors hideaway for any existing threads to see if you can pick up any more info.