We see the following event ID is logged in application log:

Log Name: Application
Source: Microsoft-Windows-User Profiles Service
Date: Date
Event ID: 1511
Task Category: None
Level: Warning
Keywords: Classic
User: User
Computer: Computer
Description:
Windows cannot find the local profile and is logging you on with a temporary profile. Changes you make to this profile will be lost when you log off.

Title: A temporary profile is loaded after you log on to a Windows Vista-based system 

Check the following article, which describes about the cause.

http://support.microsoft.com/kb/947242