browser_version= parseInt(navigator.appVersion);
browser_type = navigator.appName;

if (browser_type == "Microsoft Internet Explorer" && (browser_version >= 4) && (navigator.userAgent.indexOf('Win') != -1)) {
document.write("<LINK REL='stylesheet' HREF='styles/ie.css' TYPE='text/css'>");
} 

else if (browser_type == "Microsoft Internet Explorer" && (browser_version >= 4) && (navigator.userAgent.indexOf('Mac') != -1)) {
document.write("<LINK REL='stylesheet' HREF='styles/ie.css' TYPE='text/css'>");
} 

else if (browser_type == "Netscape" && (browser_version >= 4) && (navigator.userAgent.indexOf('Win') != -1)) {
document.write("<LINK REL='stylesheet' HREF='styles/nn.css' TYPE='text/css'>");
}

else if (browser_type == "Netscape" && (browser_version >= 4) && (navigator.userAgent.indexOf('Mac') != -1)) {
document.write("<LINK REL='stylesheet' HREF='styles/nn.css' TYPE='text/css'>");
}