I am suddenly unable to login to my account. I'd enter my username/password, but it reverts back to the login window. So I logged in using my secondary account, and used the Disk Utility to verify the disk and repair permissions. Since this did not work, I then used Onyx and ran the maintenance tools. This did not work either so I followed the suggestion given by Kappy in https://discussions.apple.com/thread/1882504. I started in single-user mode, did an fsck, deleted the files in /Library/Caches and in /System/Library Caches, as well as the com.apple.loginwindow.plist and com.apple.windowser.plist in the /Library/Preferences folder. However, the file /Systems/Library/Extensions.mkext is nonexistent so I could not delete it as Kappy recommenced. I continue to be in a login loop.