Error ORA-12560 when installing 8.1.7 on Windows 2000

When installing Oracle 8.1.7 for the first time on Windows 2000 running dual P4's (patched so it will run with the proper .dll's), I always get an ORA-12560 TNS Listener Error when trying to create my very first database. Therefore no SID is created, however, the Listener is running and can be pinged with TNSPing. Any idea what the problem is?
I'm not sure that the 12560 error is actually related to your listener at all since database creation doesn't require that a listener is running. The error text for 12560 is "TNS:protocol adapter error."

On Windows hosts, this error is usually related to the setting for SQLNET.AUTHENTICATION_SERVICES. On some installations or on some systems, this parameter (in the sqlnet.ora file) is set to NTS. On others, it is not set at all. I'd try toggling this setting and see if that changes the errors you're seeing.

I can also say that 12560 is usually not seen alone. There are frequently other errors in the stack and if that's the case for you, then investigating the other errors in the stack may provide more insight on the root cause of your problem.