After you’ve installed Windows Server 2003, you need to add the role of Application Server to your server. This will configure IIS, SMTP, etc. and you can optionally choose to configure FrontPage 2002 Server Extensions and the ASP.NET Framework. At this point, do configure the ASP.NET Framework, but do not configure FPSE 2002, even if you are going to use FrontPage to customize your portal! You don’t need the FPSEs on Win2K3 to customize SPS 2003.
If you did happen to install the FPSEs at this step, your Default Web Site will be extended with FPSEs, and appear to the SharePoint Central Administration to be a site already in production (which requires a different upgrade path). You won’t be able to extend the Default Web Site to use SharePoint unless you remove the FPSEs. You’ll know when you get this error message:
Setup has detected that your default virtual server is running FrontPage 2002 Server Extensions. To continue Setup and upgrade your default virtual server later, click OK. To exit Setup and move data from your default virtual server, click Cancel. For more information about moving data from FrontPage 2002 Server Extensions, see the Administrator’s Guide for Windows SharePoint Services.
Chances are, clicking OK isn’t going to do a darn thing. There’s a Microsoft KB article that deals with the problem if you’ve already caused it: