In SS2K , how to enable remote connections ?

For example, from my machine, within SEM I'd like to create a "New SQL
Server Registration" connection to that machine.  This has always worked for
me in the past, but today I've encountered an SS2K instance which gives me a
"SQL Server does not exist or access denied. ConnectionOpen (Connect())"
error.

Does it matter if the target instance is configured as "Windows
Authentication" vs "Mixed Mode" ?

I'm attempting the "New SQL Server Registration" via Windows Authentication.