Error message:Archival required?

I'm using Oracle 8.0.4 for NT. I have it in archive log mode, but this occurs even if I turn it off:

    After executing a few statements, or sometimes just when it is starting up, the database gives me the following message in a command-prompt window:

        Unable to allocate log: archival required. Then it hangs.

What does this mean? What am I supposed to do?

        Thanks so much for your help.

                    David