Install NET Framework and service packs for 2.x and 3.x
If IIS is not installed, do a base install (if this is external facing follow your external lock down procedures).
If the server already has IIS check the .net version mapped to the default site
Presuming your SQL server is not also a web server for exciting other exciting applications verify the default site is unused and continue to map asp.net to IIS
Run SQL Setup
Re-Run the current service pack install.