Sometimes the Windows OS may not read the user profile properly, and instead, load with a temporary profile which will look completely different. Shortly after typing the username and password, Windows tries to read and load your user profile. If it is corrupted or delayed, Windows will load with a temporary profile to give temporary access to the computer.
This is likely caused by the user profile (the files and folders located under C:\Users\user_name(Vista and above) or C:\Document and Settings\user_name(2003/XP)) being corrupted. Another possible cause is a delay in reading the user profile file due to something interferring with the boot up process, such as an Anti-Virus scan.
