As far as I know there are only a few things that could cause this behavior:
1. You modified your core wordpress files (or they were modified by someone else).
-if so, it's probably backup and reinstall time.
2. The theme you have active is using wordpress api hooks for the dashboard.
-check the functions.php file for dashboard hooks or anything else unusual
3. You may be using a plugin which is modifying the dashboard.
-deactivate your plugins one by one and see if the problem resolves.
I'd actually start with items 2 and 3 first, if there is nothing there that solves the problem I'd considering backing up and reinstalling.
Hope that helps.