By default Windows 2003 domains will lock a computer out if the incorrect password is used enough times. Sounds like someone is attempting to break into your domain through VPN, Outlook Web Access, or some other services which is exposed on the Internet.
It is very likely that there is some malware running loose on your network. We have seen the same thing happen due to virus infected machines. Take a look at my blog: Tracking down that user/computer that locks AD accounts. It could take a while to track down and correct the source of the problem if you have a very distributed environment (like we do).