Hi! You can try booting from the install disc and repairing the drive but most likely it will require and archive and install to correct the problem. Tom
I would try a test user, log into the test user and see if the issue occurs here. If it doesn't go into your main user open a finder window and go into the library folder then into the preferences folder and drag the com.apple.dock.plist to the desktop and restart the machine.
If this does not work then I work Archive and Install but this time without preserving the user and network settings. If you do this after the install you will have to go into your Mac HD after and drag your data from the previous system folder.