Windows XP implements a check to prevent you from creating a user name that is the same as the computer's name.

This will prevent problems with programs that use NetBIOS names from mistaking an 03 NetBIOS entry (user), from a 03 NetBIOS entry (the computer's Messenger service).

If you open a CMD prompt and type nbtstat -a <computer_name>, a display similar to the following is produced:

Local Area Connection:                              Node IpAddress: \[nnn.nnn.nnn.nnn\] Scope Id: \[\]                                         NetBIOS Remote Machine Name Table                                  Name                 Type        Status                                  ---------------------------------------------                                  SERVERNAME     <00>  UNIQUE      Registered                                  DOMAIN         <00>  GROUP       Registered                                  SERVERNAME     <03>  UNIQUE      Registered                                  SERVERNAME     <20>  UNIQUE      Registered                                  DOMAIN         <1E>  GROUP       Registered                                  USERNAME       <03>  UNIQUE      Registered                                  MAC Address = 00-B0-D0-DC-E5-87

00 is the Workstation service.

03 is the Messenger service.
03 is the user name. 

20 is the Server service.

Notice how a duplicate user name and computer name would both register identical 03 entries.