In an effort to reduce the attack surface of your Web server, IIS 6.0 serves only static content after a default installation. Programmatic functionality provided by ISAPIs or Common Gateway Interfaces (CGIs) must be manually enabled by an IIS administrator. ISAPIs and CGIs extend the ability of your Web pages and for this reason, ISAPIs and CGIs are referred to here as Web service extensions. For example, in order to run Active Server Pages (ASP) with this version of IIS, the ISAPI Asp.dll must be enabled as a new Web service extension.
If you chose to allow all IIS Application Program Interfaces (ISAPIs) to run on your Web server, your Web server could be susceptible to computer viruses or worms that exploit ISAPI technology. Instead, use the Web Service Extensions node in IIS Manager to select only those ISAPIs that are allowed to run on your Web server
To learn more about Web service extensions and configuring applications, see the IIS 6.0 online documentation on the Microsoft Windows Server TechCenter.