According to Oracle's certification matrix, Oracle8i is not certified to run on Windows XP. Only Oracle 9.2 and onward is certified to run on Windows XP Professional, so you might want to install release 9.2 instead of 8i, if possible, so that you are using a supported database release.
In any case, the problem you are likely having is that Oracle is attempting to start the service but for some reason, it is failing and the database, consequently, does not open. This is why you cannot log in via SQL*Plus. Check the oradim.log for any error messages. If the service cannot start, it will likely send error messages to this log file.
If there are no error messages in the oradim.log file, check the database alert log for error messages. It is possible that the service is starting successfully but the database is not opening. If that is the case, the alert log should contain messages to assist you in troubleshooting and resolving this.
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