Hi Cookie
I occasionally get this if one of my External HD's is left on or if I have left a device +(iPod, iPhone, FlashDrive, Camera etc.)+ connected during restart and startup.
1. Repair your Startup Disk with your Install DVD >
http://kb.wisc.edu/helpdesk/page.php?id=3810
2. Go to: System Preferences > Startup Disk > select the Mac OS X on Macintosh HD > Lock the Lock.
Then if your still having the problem:
1. Eject and turn off any External Hard Drives
2. Eject and disconnect flash memory devices prior to shutting down and reconnect them after startup.
Another consideration if you must keep devices connected for whatever reason, is to sleep the iMac instead of starting up every morning.
Dennis