I needed to install Microsoft SQL 2000 Server Reporting Services on a new laptop and I couldn't get past the system check screen because the installation program kept telling me that ASP.NET was not registered with IIS.  I ran the aspnew_regiis tool as follows:

%windir%\Microsoft.Net\Framework\v1.1.4322 folder\aspnet_regiis -i

No dice. I still couldn't get past the system check screen.  After nearly going insane, I finally figured-out what was wrong.  I opened-up the IIS Management Console and checked the ASP.NET tab that is added once the .NET 2.0 Framework is installed.  Sure enough, the default ASP.NET version specified was version 2.0 and the SRSS installation program was checking for version 1.1.  I changed the ASP.NET version to 1.1, restarted the installation, and away I went!