To detect the Operating System on the Client Machine, your script should analyze the navigator.appVersion string. Below is a simple example of a script that sets the variable OSName to reflect the actual client OS.paste this script after the <Title> Tag of the <Head> Tag of the Page.
1: <script language="javascript" type="text/javascript">
2:
3: // This script sets OSName variable as follows:
4: // "Windows" for all versions of Windows
5: // "MacOS" for all versions of Macintosh OS
6: // "Linux" for all versions of Linux
7: // "UNIX" for all other UNIX flavors
8: // "Unknown OS" indicates failure to detect the OS
9:
10: var OSName="Unknown OS";
11: if (navigator.appVersion.indexOf("Win")!=-1) OSName="Windows";
12: if (navigator.appVersion.indexOf("Mac")!=-1) OSName="MacOS";
13: if (navigator.appVersion.indexOf("X11")!=-1) OSName="UNIX";
14: if (navigator.appVersion.indexOf("Linux")!=-1) OSName="Linux";
15:
16: document.write('Your OS: '+OSName);
17:
18: </script>
On your system, this script yields the following result: Your OS: Windows
(To get more detailed OS information, your script should perform a more sophisticated analysis of the navigator.appVersion string, but the idea would be the same. 
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