Fix?
I had the same problem (After uninstalling Windows to try and enlarge the disk)
I called Apple, and all you have to do is
1: Boot from the Apple Install CD that came with your comp. Hold C while rebooting. The Apple logo should appear and wait a few minutes.
2: Choose your language in the menu
3: One of the items in the one of the menus of the menu bar has "Disk Utility"
4: Select your Macintosh HD. Click Repair Permissions, wait, Click Repair Disk, wait, Click Repair Disk again and make sure that it says "successful"
It should work now!