if (window.navigator.userAgent.indexOf("MSIE")>=1) 
{  
var IE800="style2.css"; 
var IEother="style1.css"; 
ScreenWidth(IE800,IEother) 
} 
else{ 
if (window.navigator.userAgent.indexOf("Firefox")>=1) 
{ 
var Firefox800="style2.css"; 
var Firefoxother="style1.css"; 
ScreenWidth(Firefox800,Firefoxother) 
} 
else{ 
var Other800="style2.css"; 
var Otherother="style1.css"; 
ScreenWidth(Other800,Otherother) 
} 
}

function ScreenWidth(style2,style1){ 
if ((screen.width == 800) && (screen.height == 600)){ 
setActiveStyleSheet(style2); 
}else{ 
setActiveStyleSheet(style1); 
}
} 

function setActiveStyleSheet(title){ 

document.getElementsByTagName("link")[1].href="../css/"+title; 
}