That you're giving it an instance name. If you already have SQL Server, (or a default instance of MSDE, for that matter) it obviously won't let you install another default instance. You'll have to give it a unique name.
I tryed installing MSDE, launched the setup.exe inside the MSDE folder of the SQLServer CD. It didn't prompt anything.. so I don't know what it actually did but I doubt it gave the MSDE an instance name!<BR>Now, on startup, I get this message:<BR>"Your SQL Server installation is either corrupt or has been tampered with (unknown package id). Please uninstall then re-run setup to correct this problem."<BR><BR>T__T<BR><BR>