Accounts locked out

We are having some major problems on our network with accounts being locked out. At first, it was just our users that were getting locked out randomly. Over last night, the web server accounts such as the anonymous user account were locked out! So our web site was down! In fact, the web site is on a different server than the users, and every user account on the web server was locked out.

Anyone know how we can prevent this? We are using Windows 2000 Server with Active Directory. Do you think it has something to do with DNS? Any help will be appreciated!

Casey