This was happened twice at my office and at the same PC. Have no idea what is the cause.
But the solution is right here http://support.microsoft.com/kb/947215
When you log on to a Windows 7-based or a Windows Vista-based computer by using a temporary profile, you receive the following error message:
The User Profile Service failed the logon. User profile cannot be loaded.
Occasionally, Windows might not read your user profile correctly, such as if your antivirus software is scanning your computer while you try to log on. Before you follow the methods here, try restarting your computer and logging on with your user account again to resolve the issue. If you restart your computer and it does not resolve this issue, use the following methods to resolve this issue.
Note You must be able to log on to an administrator account to fix your user profile or copy your data to a new account. Before you resolve the issue, log on to Windows by using another account that has administrative permissions, or restart in safe mode to log on with the built-in administrator account.
(I did without any administrator account, and it still worked!)
Method 1: Fix the user account profile
To fix the user account profile, follow these steps:
Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs.
1. Click Start, type regedit in the Search box, and then press ENTER.
2. In Registry Editor, locate and then click the following registry subkey:
3. In the navigation pane, locate the folder that begins with S-1-5 (SID key) followed by a long number.
4. Click each S-1-5 folder, locate the ProfileImagePath entry in the details pane, and then double-click to make sure that this is the user account profile that has the error.
(If you want better illustration with image, just click this link http://support.microsoft.com/kb/947215
#If you have two folders starting with S-1-5 followed by some long numbers and one of them ended with .bak, you have to rename the .bak folder. To do this, follow these steps:
a.Right-click the folder without .bak, and then click Rename. Type .ba, and then press ENTER.
b.Right-click the folder that is named .bak, and then click Rename. Remove .bak at the end of the folder name, and then press ENTER.
c.Right-click the folder that is named .ba, and then click Rename. Change the .ba to .bak at the end of the folder name, and then press ENTER.
#If you have only one folder starting with S-1-5 that is followed by long numbers and ends with .bak.
5. Right-click the folder, and then click Rename. Remove .bak at the end of the folder name, and then press ENTER.
6. Click the folder without .bak in the details pane, double-click RefCount, type 0, and then click OK.
7. Close Registry Editor.
8. Restart the computer.
9. Log on again with your account.