I recommend RENAMING the account instead of deleting it; this will also make
it much harder for an intruder to guess the name. BTW, just out of curiosity
what is your motive for trying to delete it?
If you need to use an Administrator account on a daily basis, then leave the
one you created. Keep the built in one for emergencies, in case you can't
get in on your profile. However, if you really want to remove
it............
For XP PRO: Press CTRL-ALT-DEL twice at the Welcome Screen, with no users
logged in. You'll see the old Win2K style logon. Enter Administrator and
any password that's been set for the built in account. You can now delete
the account you created from Control Panel, User Accounts.
For XP Home: Boot the computer into Safe Mode (See Help and Support).
You'll see the built in Administrator account listed on the Welcome Screen.