Log into another (admin) user (if you have one). Open /Applications/Utilities/Terminal. Type this:
sudo mv /Users/your_short_username/Library/Preferences/com.apple.loginwindow.plist /Users/your_short_username/Desktop/
I put it in code tags because it's one very long line. Naturally, replace "your_short_username" with your short username.
Logout and try logging in as your normal user.