Does it still show the old SID in the Services window after you delete it?
Check to see if the password file is also deleted when you delete the service with oradim80 -delete.
The password file is created when you create the instance with oradim80 -new and is located (if I remember exactly) in the %ORACLE_HOME%\Database folder and its name is pwdSID.ora where SID is the name of your instance created with oradim80, pwdPROD.ora in your case.
So, after you issue -oradim80 -delete etc. check to see if the password file was deleted and if not delete it manually. If I remember correctly if the password file exists the service will not be created.

Hope this help.