There are lots of reasons why you can get the TNS-12203 error, but the good news is that 99% of the time it is because you've hosed you configuration files - which is an easy fix.

As an aside, 7.1 is not supported on Windows98.

Check your tnsnames.ora and listener.ora files for correct spelling and format. You should also have a wsock32.dll file sized around 40K located in windows\system.