Database Opened in Exclusive Mode

I just can't figure this out - what am I doing here? I am starting my instance in the Oracle Instance Manager but it will not let me open it (Cannot open database in exclusive mode). The manual says that this is because another instance has already mounted the database. But where is this other instance, how do I get to it and how do I shut it down? I only seem to be able to shutdown the instance that I've just started up. I am running Oracle8 on a stand-alone machine.

Any help gratefully received,

Thanks,

• Bill