Este artigo pode ajudar:
The jury is still out on the exact cause, but so far :
- This issue may occur if the user profile was manually deleted by using the command prompt or Windows Explorer by a user or by some program. A profile that is manually deleted does not remove the security identifier (SID) from the user profile list in the registry. Since the SID is still present, Vista will still try to load the profile by using the ProfileImagePath that points to a nonexistent path. Therefore, the profile cannot be loaded.
- This can also be a issue with the user profile entering into a backup state.
- Manually renamed the C:\Users(User Name) folder.
O artigo lista vários métodos para corrigir o problema.