password lockout in a domain

Hi all!
i know this question has been discussed previously, but i had this problem many times today
i changed the password policy on the domain and account lockout policy was not defined at all.
I forced users to change password on next logon.out of 2000 users , i got abt 30 users whose account got locked out, some of the users infact 3-4 of them were getting locked almost evry 15 min
can anyone help me in this..

also i have one more question, we are running sharepint portal server 2003, is the autentication on sharepoint server linked to AD in anyways, cos evryone was prompted to put the password when accessing the portal server

thanx a lot