function checkLinux() {
	var is_linux = "is_linux";
	var fRet = true;
	if (!checkOS(is_linux)) { 
		fRet = confirm("You're not on a Linux computer right now... Are you sure you want to download the Linux client?"); 
	}
	return fRet;
}

function checkWindows() {
	var is_win = "is_win";
	var fRet = true;
	if (!checkOS(is_win)) {
    	fRet = confirm("You're not on a Windows computer right now... Are you sure you want to download the Windows client?"); 
	}
	return fRet;
}

function checkMac() {
	var is_mac = "is_mac";
	var fRet = true;
	if (!checkOS(is_mac)) {
    	fRet = confirm("You're not on a Mac computer right now... Are you sure you want to download the Mac client?"); 
	}
	return fRet;
}

function checkOS(is_system) {
	var agt=navigator.userAgent.toLowerCase();

	// *** PLATFORM ***
    var is_win = (agt.indexOf("win")!=-1);
               
    // NOTE: On Opera 3.0, the userAgent string includes "Windows 95/NT4" on all
    //        Win32, so you can't distinguish between Win95 and WinNT.
    var is_win95 = ((agt.indexOf("win95")!=-1) || (agt.indexOf("windows 95")!=-1));

    // is this a 16 bit compiled version?
    var is_win16 = ((agt.indexOf("win16")!=-1) || 
               (agt.indexOf("16bit")!=-1) || (agt.indexOf("windows 3.1")!=-1) || 
               (agt.indexOf("windows 16-bit")!=-1) );  

    var is_win31 = ((agt.indexOf("windows 3.1")!=-1) || (agt.indexOf("win16")!=-1) ||
                    (agt.indexOf("windows 16-bit")!=-1));

    var is_winme = ((agt.indexOf("win 9x 4.90")!=-1));
    var is_win2k = ((agt.indexOf("windows nt 5.0")!=-1));

    // NOTE: Reliable detection of Win98 may not be possible. It appears that:
    //       - On Nav 4.x and before you'll get plain "Windows" in userAgent.
    //       - On Mercury client, the 32-bit version will return "Win98", but
    //         the 16-bit version running on Win98 will still return "Win95".
    var is_win98 = ((agt.indexOf("win98")!=-1) || (agt.indexOf("windows 98")!=-1));
    var is_winnt = ((agt.indexOf("winnt")!=-1) || (agt.indexOf("windows nt")!=-1));
    var is_win32 = (is_win95 || is_winnt || is_win98 || 
                    (navigator.platform == "Win32") ||
                    (agt.indexOf("win32")!=-1) || (agt.indexOf("32bit")!=-1));

    var is_os2   = ((agt.indexOf("os/2")!=-1) || 
                    (navigator.appVersion.indexOf("OS/2")!=-1) ||   
                    (agt.indexOf("ibm-webexplorer")!=-1));

	var is_mac    = (agt.indexOf("mac")!=-1);
    // hack ie5 js version for mac
    if (is_mac && is_ie5up) is_js = 1.4;
    var is_mac68k = (is_mac && ((agt.indexOf("68k")!=-1) || 
                               (agt.indexOf("68000")!=-1)));
    var is_macppc = (is_mac && ((agt.indexOf("ppc")!=-1) || 
                                (agt.indexOf("powerpc")!=-1)));

    var is_sun   = (agt.indexOf("sunos")!=-1);
    var is_sun4  = (agt.indexOf("sunos 4")!=-1);
    var is_sun5  = (agt.indexOf("sunos 5")!=-1);
    var is_suni86= (is_sun && (agt.indexOf("i86")!=-1));
    var is_irix  = (agt.indexOf("irix") !=-1);    // SGI
    var is_irix5 = (agt.indexOf("irix 5") !=-1);
    var is_irix6 = ((agt.indexOf("irix 6") !=-1) || (agt.indexOf("irix6") !=-1));
    var is_hpux  = (agt.indexOf("hp-ux")!=-1);
    var is_hpux9 = (is_hpux && (agt.indexOf("09.")!=-1));
    var is_hpux10= (is_hpux && (agt.indexOf("10.")!=-1));
    var is_aix   = (agt.indexOf("aix") !=-1);      // IBM
    var is_aix1  = (agt.indexOf("aix 1") !=-1);    
    var is_aix2  = (agt.indexOf("aix 2") !=-1);    
    var is_aix3  = (agt.indexOf("aix 3") !=-1);    
    var is_aix4  = (agt.indexOf("aix 4") !=-1);    
    var is_linux = (agt.indexOf("inux")!=-1);
    var is_sco   = (agt.indexOf("sco")!=-1) || (agt.indexOf("unix_sv")!=-1);
    var is_unixware = (agt.indexOf("unix_system_v")!=-1); 
    var is_mpras    = (agt.indexOf("ncr")!=-1); 
    var is_reliant  = (agt.indexOf("reliantunix")!=-1);
    var is_dec   = ((agt.indexOf("dec")!=-1) || (agt.indexOf("osf1")!=-1) || 
           (agt.indexOf("dec_alpha")!=-1) || (agt.indexOf("alphaserver")!=-1) || 
           (agt.indexOf("ultrix")!=-1) || (agt.indexOf("alphastation")!=-1)); 
    var is_sinix = (agt.indexOf("sinix")!=-1);
    var is_freebsd = (agt.indexOf("freebsd")!=-1);
    var is_bsd = (agt.indexOf("bsd")!=-1);
    var is_unix  = ((agt.indexOf("x11")!=-1) || is_sun || is_irix || is_hpux || 
                 is_sco ||is_unixware || is_mpras || is_reliant || 
                 is_dec || is_sinix || is_aix || is_linux || is_bsd || is_freebsd);

    var is_vms   = ((agt.indexOf("vax")!=-1) || (agt.indexOf("openvms")!=-1));
    
    var fRet;
    if (is_system == "is_linux") {
    	fRet = is_linux;
	}
	
	if (is_system == "is_win") {
		fRet = is_win;
	}
	
	if (is_system == "is_mac") {
		fRet = is_mac;
	}
	
	return fRet;
}