if (/MSIE (\d+\.\d+);/.test(navigator.userAgent)){ //test for MSIE x.x;
var ieversion=new Number(RegExp.$1) // capture x.x portion and store as a number
if (ieversion<=6)
  document.write("<link href=\"style_sheets/stylesheet2.css\" rel=\"stylesheet\" />");
 else if (ieversion>=7)
  document.write("<link href=\"style_sheets/stylesheet1.css\" rel=\"stylesheet\" />");
 
}
else
 document.write("<link href=\"style_sheets/stylesheet1.css\" rel=\"stylesheet\" />");