I've found this key here:
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\User Shell Folders
You can see that the values dont list a specific username, but %USERPROFILE%. The computer would use this key to load the start menu of which ever user is logged on, in this case it should be your user, but it isn't.
This tells me that your user profile (or atleast your start menu profile) may not exist anymore, thus the computer reverts to "Default User".
To check if your start menu profile still exists, open up windows explorer and navigate to:
C:\Documents and Settings\YOURUSERNAME
If it says file not found etc, then obviously your profile is gone. If it is there proceed to the next step.
After checking that your start menu still exists, you may want to try to change the values in HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\User Shell Folders
from "%USERPROFILE%" to your username.
Edited by Da.One, 20 February 2008 - 12:08 PM.