|No answer ??|
So I will take a stab at it. I would guess that that account is a Customized Default User Account. You might look in D & S to see if you a 'Default User' showing there.
C:\Documents and Settings\Default User
If you do not, then the one you cannot delete is a Customized Default User Account.
Windows Help says ---
The profile that serves as a basis for all user profiles. Every user profile begins as a copy of the default user profile.
How to create a custom default user profile