BAS is available only under Win2003 and depends on the SharePoint Services (SPS). You even cannot check it for the installation if SPS was not already installed. So first of all, you need to obtain the SPS. SPS is not the same thing as SharePoint Portal Server and it is available for free download from MS site.
Once you downloaded and installed it (or you already have SharePoint Portal Server), you can to select BAS for the installation, but before doing so, keep in mind that BAS primary site will be installed on the virtual server with applied SPS extensions. So you you need to configure your IIS before continuing BAS installation.
If you already running the IIS with FrontPage Server Extensions, this procedure is a bit complex:
1). backup your server : run smigrate.exe from C:Program FilesCommon FilesMicrosoft Sharedweb server extensions60BIN like
smigrate -w http://localhost -f backup.fwp.
This may take several hours depending on size of the sites.
2). remote existing FPExtension : run owsadm.exe from C:Program FilesCommon FilesMicrosoft Sharedweb server extensions50bin like
owsadm.exe -o fulluninstall -p (usually 80)
3). extend your virtual server to SPS extension : launch your SPS Admin site, navigate to "extend virtual server", select your server
- When using Windows SharePoint Services with SharePoint Portal Server, re-use the MSSharePointPortalServerAppPool application pool created by SharePoint Portal Server. This application pool can be managed by clicking on Configure Server Farm Account Settings in SharePoint Portal Server Central Administration.
3a). Alternatively, you can use stsadm.exe from C:Program FilesCommon FilesMicrosoft Sharedweb server extensions60BIN directory
4). restore your IIS site configuration from the step 1). : run smigrate.exe once again noe specifying -r option like
smigrate.exe - r -w http://localhost -f backup.fwp
This also may take several hours depending on size of the sites.
IF YOU WERE HAPPY TO ACCOMPLISH ALL OF THIS, YOU CAN CONTINUE BAS CONFIGURATION! :)