Originally posted by übergeek
you can pop in your OS X install disks and open disk utility and fix permissions from there (look for em in the menu, i know its there)
On the other hand, you can boot up in single user mode (reboot machine and hold down apple+s when you see the gray screen you get before you see the Apple logo) and run fsck by typing in "fsck -yf" at the prompt which should look something like "root@localhost: #" or something like that (i got customized prompts so it will definately vary)
If you're running Panther it will tell you to type something like "/sbin/fsck -y" if you want to make modifications, that's basically what you need to type, except you need to add the f flag (if you do it without and your hard drive is journaled it'll tell you to do so).
Keep running fsck until it tells you that your hard drive seems to be okay.
If those two ways don't work, ask around some more, then reinstall. Good luck 😛