How can I know the free memory (free RAM) at a moment with
a JScript.

One way to do it:

//////////////////////////////////////////////////////////////////////
//    File: Mem.js

var oShell = new ActiveXObject("WScript.Shell");

if (!isCScript()||(WScript.Arguments.Length!=1)) {
    showUsage();
    WScript.Quit();

var sHost = WScript.Arguments(0);   //  sHost can be Host Name or IP address
showMemInfo(sHost);

//////////////////////////////////////////////////////////////////////

function showMemInfo(sHost) {

    try {
        var oOSset = GetObject("winmgmts:{impersonationLevel=impersonate}//" + sHost).InstancesOf ("Win32_OperatingSystem")
    } catch(e) {
        switch (e.Number) {
            case 462 :
                //  The remote server machine does not exist or is unavailable
                WScript.Echo ("Host is not available.");
                return;
            case 429 :
                //  ActiveX component can't create object
                WScript.Echo ("Host is missing WMI capabilities.");
                return;
            case -2147217405 :
                //  Lacking admim permissions to connect to host
                WScript.Echo("Not enough permissions to connect to this host.");
                return;
            default :
                //  Whatever else the cause may be...
                WScript.Echo(e.description);
                return;
        }
    }

    var oEnm_1 = new Enumerator(oOSset);
    var oEnm_2;

    WScript.Echo();
    WScript.Echo("Host: " + sHost);

    for (; !oEnm_1.atEnd(); oEnm_1.moveNext()) {
        oEnm_2 = new Enumerator(oEnm_1.item().Properties_);

        for (; !oEnm_2.atEnd(); oEnm_2.moveNext()) {
            //  Pick only memory related properties (ignore the rest):
            switch (oEnm_2.item().Name) {
                case "TotalVisibleMemorySize" :
                    WScript.Echo("\tTotal RAM         -> " + Math.round(oEnm_2.item().Value/1024) + "\tMB");
                    continue;
                case "FreePhysicalMemory" :
                    WScript.Echo("\tFree RAM          -> " + Math.round(oEnm_2.item().Value/1024) + "\tMB");
                    continue;
                case "FreeSpaceInPagingFiles" :
                    WScript.Echo("\tFree in page file -> " + Math.round(oEnm_2.item().Value/1024) + "\tMB");
                    continue;
                case "TotalVirtualMemorySize" :
                    WScript.Echo("\tTotal virtual mem -> " + Math.round(oEnm_2.item().Value/1024) + "\tMB");
                    continue;

            }
        }
    }

function isCScript() { return (/cscript.exe$/i).test(WScript.FullName); }

function showUsage() {
    var sMsg = "Syntax: \tC:\\>CScript //nologo " + WScript.ScriptName + " IP\r\n";
    sMsg += "        \t(must be run by 'CScript.exe')";
    WScript.Echo(sMsg);

// Branimir