login attempts

From: Pirillo (ftachi@tin.it)
Date: 11/02/02 

From: "Pirillo" <ftachi@tin.it>
Date: Sat, 2 Nov 2002 11:20:34 +0100

Every day i have on my win2000 iternet server a lots of wrong login attempts
from anonymous workstation. Someone try to login with useful name such as
"admin" "asdf" etc.
Recently they have used server existent login names and try a lot of
password. They lock the account.
How have they obtained the list of login names.
What are the correct operation to prevento and resolve this situation.
Can i know from where are caming this attacks ?
Thanks to all