Administrator Account Frequently Locked

I have been noticing recently in my NT 4.0 Security logs that my Administrator account is constantly being locked out. It appears from the security event log that repeated failed login attempts from a domain and workstation unknown to me is occuringfrequently. This is not a service on one of my servers causing this. This is occurring frequently enough that I am concerned this is a hacker playing guess the password. It’s also frequent eoungh that the account might be locked when I need to use it.
My questions are: can I block login attempts from other domains/workstations somehow? Are there other suggestions that I might try?