I tried to install Oracle 7.1 for windows on my Windows 98 machine. Installation was shown successfull. But logging in to SQL PLUS with Username/Pwd, Scott/Tiger was showing the error " ERROR ORA-12203 : (Cnct err, Can't get err txt. see Server Msgs & Codes manual) ". Could you please let me know whether i need to modify Oracle.ini file to create the Host String.
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.