jsHeader:

var f;

function GetUserLogon(){
var objUser = new ActiveXObject("WScript.Network");
f.LogonID.value=objUser.UserName;
f.ComputerName.value=objUser.ComputerName;
//f.DomainName.value=objUser.DomainName;
}


onLOad:
f = document._tmp;
GetUserLogon();


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Note: As this script is "unsigned" and not "marked safe", this may fail in real-life use.