Frank Heyne has released EventSave which is a Windows NT and Windows 2000 freeware utility which automates saving and deleting of event logs.
Quote from site:
There is no utility delivered with Windows NT to move the events from the current logs into backup files.
I wrote a little tool which can be used to schedule a job to automatically save all events to another file and clear each log afterwards. Independently of how often you run it, there will be one file created every month for each log, called year_month_computer_xxx.evt, where xxx is the name of the log (like, for instance, Security or System).