I had a customer who was working in his Dell computer running Vista home premium. Internet Explorer crashed and the next time he logged out, when he logged in he got the error:
The User Profile Service failed the logon. User profile cannot be loaded.
After this Vista then logs out. Luckily Vista will load up into Safe Mode and allow the user's account to be logged into. The administrator account is disabled by default and is not available. The next step is to start regedit by typing it in the search input box in the start menu. Using the tree on the left hand side navigate to the following:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList
The tree below this will consist of one or more items consisting of a series of letters and characters like in the following screen shot


In this case one of the profile items ended with a .bak. Double clicking on the item, edit the key and remove the .bak suffix. Exit and reboot and the system should allow the user to login.