It seems that you are unaware of the basic security traits
of Windows NT/2000/XP. This class of operating systems
is "secure", meaning that only people with proper credentials
can log on. In other words, everyone must have an account
name and a password. What you see is not an "error
message", as you call it. It is a message to inform you that
the account name and password you entered are incorrect.
They do not agree with any of the account names/passwords
that are currently stored in the system.
It is possible to arrange things under Win2000 so that the
system logs you on automatically. This does NOT mean
that there is no account name or password; it simply means
that the system supplies these details automatically at
logon time. It now appears that this automatic mechanism
got disturbed on your PC. It also appears that you never
took care to write down the details of the administrator's
account, and to keep them in a safe place. It's like buying
a new car and not keeping one or two sets of spare keys.
I recommend strongly that you change your ways in
order to protect yourself against similar mishaps. After
resetting the administrator's account, make a note of
its password. Create a second administrative account,
with a password, and lock these details away in a place
where you can easily find them.
To reset the administrator's password, boot the machine
with a disk from the link below. It's scary stuff but it
works.
http://home.eunet.no/~pnordahl/ntpasswd/bootdisk.html