.NET virus scanning API

You can use IAttachmentExecute API.

Windows OS provide the common API to calling the anti virus software which is installed (Of course, the anti virus software required support the API). But, the API to calling the anti virus software provide only COM Interface style, not supported IDispatch. So, calling this API is too difficult from any .NET language and script language.

Download this library from here Anti Virus Scanner for .NET or add reference your VS project from "NuGet" AntiVirusScanner

For example bellow code scan a file :

var scanner = new AntiVirus.Scanner();
var result = scanner.ScanAndClean(@"c:\some\file\path.txt");
Console.WriteLine(result); // console output is "VirusNotFound".

Important note before use: Be aware of TOS agreement. You give them full access to everything: "When you upload or otherwise submit content, you give VirusTotal (and those we work with) a worldwide, royalty free, irrevocable and transferable licence to use, edit, host, store, reproduce, modify, create derivative works, communicate, publish, publicly perform, publicly display and distribute such content."

Instead of using a local Antivirus program (and thus binding your program to that particular Antivirus product and requesting your customers to install that Antivirus product) you could use the services of VirusTotal.com

This site provides a free service in which your file is given as input to numerous antivirus products and you receive back a detailed report with the evidences resulting from the scanning process. In this way your solution is no more binded to a particular Antivirus product (albeit you are binded to Internet availability)

The site provides also an Application Programming Interface that allows a programmatically approach to its scanning engine.

Here a VirusTotal.NET a library for this API
Here the comprensive documentation about their API
Here the documentation with examples in Python of their interface

And because no answer is complete without code, this is taken directly from the sample client shipped with the VirusTotal.NET library

static void Main(string[] args)
{
    VirusTotal virusTotal = new VirusTotal(ConfigurationManager.AppSettings["ApiKey"]);

    //Use HTTPS instead of HTTP
    virusTotal.UseTLS = true;

    //Create the EICAR test virus. See http://www.eicar.org/86-0-Intended-use.html
    FileInfo fileInfo = new FileInfo("EICAR.txt");
    File.WriteAllText(fileInfo.FullName, @"X5O!P%@AP[4\PZX54(P^)7CC)7}$EICAR-STANDARD-ANTIVIRUS-TEST-FILE!$H+H*");

    //Check if the file has been scanned before.
    FileReport fileReport = virusTotal.GetFileReport(fileInfo);

    bool hasFileBeenScannedBefore = fileReport.ResponseCode == ReportResponseCode.Present;

    Console.WriteLine("File has been scanned before: " + (hasFileBeenScannedBefore ? "Yes" : "No"));

    //If the file has been scanned before, the results are embedded inside the report.
    if (hasFileBeenScannedBefore)
    {
        PrintScan(fileReport);
    }
    else
    {
        ScanResult fileResult = virusTotal.ScanFile(fileInfo);
        PrintScan(fileResult);
    }
    ... continue with testing a web site ....

}

DISCLAIMER
I am in no way involved with them. I am writing this answer just because it seems to be a good update for these 4 years old answers.