NT 5.0 and IIS support SSL and NT Authentication (if you don't want to use a custom solution based off your SQL database).
My suggestion since you have the equipment and hardware is to look at
ASPin and
HotScripts in the ASP section. You will find full featured e-commerce applications geared towards a SQL Server database. You will find Security Solutions. You will also find many more solutions geared towards your current architecture.
Second thing to do is learn ASP since its running on your current architecture. Pick up "Professional ASP 3.0 Programming" from Wrox Press. You can also visit sites such as ASP101.COM, 4GUYSFROMROLLA.COM and ASPTODAY.COM for online tutorials and examples
Third, read your IIS help files, they can tell you how to install SSL security on your site and use NT Authentication for access.