You don't have to uninstal Oracle just for that.
That's what I would do:
-shutdown abort if the instance in still started;
- delete the service using oradim80 -delete -sid ORCL(or the name of your sid ifis different from ORCL)
-delete all database files, control files, redo log.
Now there is no trace of your previous database in the system. You should be able to recreate it from scratch