You can not accomplish it with ASP. As ASP is a scripting language and no system progrmming could be done with it (system interaction), however if you want to do some operations with WinAPI, you'll need to build an ActiveX DLL and call it's public function from your ASP code.
In current scenario, where you need to authenticate a user with Active Directory, you'll need to develop an ISAPI filter for IIS. This filter shall be activated on whole of the IIS or on a site, where you need to enable this functionality.
Important: An ISAPI filter could not be programmed in VB6, but you'll have to develop it with VC++ or .NET.
Following are some URL, which will give a brief overview of ISAPI filters...
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ISAPI authentication filter
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ISAPI filter to allow access only to hosts with selected IP addresses
You can also search for "ISAPI filter" at
http://www.codeproject.com to find more examples.