window.onresize = sirka;
window.onload = sirka;

var wid;
var oldwid;
function sirka(){    
  if  (document.documentElement && document.documentElement.scrollTop)
	     document.documentElement.clientWidth; //ie6 with doctype
  else if (document.body)
        wid =document.body.clientWidth; // ie5, 6, ns 6
    if(wid>0 && wid < 800) {     
      wid =800;
      oldwid =wid;
      document.getElementById("horni").style.width = wid;
      document.getElementById("hlavni").style.width = wid; 
      }
    else if(oldwid==800 && wid >800){
      document.getElementById("horni").style.width = wid-40;
      document.getElementById("hlavni").style.width = wid-40; 
      }
       }