var Win=(navigator.userAgent.indexOf("Win")!=-1);
var Mac=(navigator.userAgent.indexOf("Mac")!=-1);
var Explorer=(navigator.appName.indexOf("Explorer")!=-1);
var Firefox=(navigator.userAgent.indexOf("Firefox")!=-1);
var Netscape=(navigator.appName.indexOf("Netscape")!=-1);
var Safari=(navigator.userAgent.indexOf("AppleWebKit")!=-1);

if
(Win && Explorer)
{document.write('<link rel="stylesheet" href="styles/os/win_ie.css" type="text/css">');}

else if
(Win && Firefox)
{document.write('<link rel="stylesheet" href="styles/os/win_ff.css" type="text/css">');}

else if
(Win && Netscape)
{document.write('<link rel="stylesheet" href="styles/os/win_nn.css" type="text/css">');}

else if
(Mac && Safari)
{document.write('<link rel="stylesheet" href="styles/os/mac_sa.css" type="text/css">');}

else if
(Mac && Firefox)
{document.write('<link rel="stylesheet" href="styles/os/mac_ff.css" type="text/css">');}

else if
(Mac && Netscape)
{document.write('<link rel="stylesheet" href="styles/os/mac_nn.css" type="text/css">');}

else if
(Mac && Explorer) 
{document.write('<link rel="stylesheet" href="styles/os/mac_nn.css" type="text/css">');}

else
{document.write('<link rel="stylesheet" href="styles/os/win_ie.css" type="text/css">');}