After installing some Windows updates on Windows Vista or Windows 7 PC you may not be able to log-on and get error message:
The User Profile Service service failed the logon.
User profile cannot be loaded.
1. Restart computer in safe mode (F8 on boot) and you should be able to login.
2. Start > regedit
3. Navigate to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList
4. There will be number of Keys. You need to find one which belongs to your broken profile. You can check "ProfileImagePath" value for each key to find out which one belongs to your profile.
5. Let's say your profile key is named "S-1-5-21-2947451878-954321858-1958354267-4471" There will be another Key called "S-1-5-21-2947451878-954321858-1958354267-4471.bak" (the same with .bak at the end)
6. Right click on the first key and add .bak2 at the end.
7. Right click on the second key and remove .bak from the end.
8. Right click on the first key again and change .bak2 to .bak
9. Restart your computer
This should sort the issue (worked for me). If this doesn't help you can find more information and suggestions here.