Not normally. There is already a built-in Administrator account, which
cannot be deleted. It can be renamed, however, and then you might be
able to rename your account to "Administrator." However, the user
profile folders (C:\Documents and Settings\Username) _cannot_ be
renamed, without risky registry edits, even if the associated user
account has been. So, your best course of action would be to log on
using the built-in Administrator account, create a new user account,
with the username desired. You can then delete the old user account(s),
after copying the desired data from the old to the new.
HOW TO Create and Configure User Accounts in Windows XP
http://support.microsoft.com/default.aspx?scid=kb;en-us;279783
How to Copy User Data to a New User Profile
http://support.microsoft.com/default.aspx?scid=kb;en-us;811151
What, precisely, are you actually trying to accomplish?