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I would hope that the easiest way out of your dilemma is a system restore to a point before the attempt was made to change the permissions configuration.
You may need to go into Safe Mode to do it though.
Also, creating a new user account, and then deleting the old one may do the trick, but I'm not so sure on that one.
Not sure either whether this tutorial will help, but I'll post the link anyway. Sharing and Permissions