Macintosh HD will open from

Sounds like you may have some permissions issues - I would suggest sticking in the OS disc that came with your iMac, restarting, and holding the "C" key when you hear the chime - this will make it boot off the CD. Once booted from it, go to Tools->Disk Utility, choose your disk, and do "Repair Permissions". Once that is done, you can quit the installer and restart. If that doesn't work, the next step I would recommend is running DiskWarrior, which is $99 - this is like an industrial permissions/hard drive directory repair disc, used by many professionals. If that doesn't solve it, you will likely have to back up your user data and do a fresh install of the OS from the discs you have. Good luck!