Penso, che tu crei ad ogni pressione un iframe....

function prova() {
document.body.innerHTML+="<div id='iframe' name='iframe' style='display:visible;' ><iframe id='prova' name='prova' style='display:visible;position:absolute; top: 20px; right: 350px;' src='window.html' height='520' width='350' FRAMEBORDER='0' SCROLLING='NO'></div>";
}


Se fai

function prova() {
if (top.frames["iframe"] == null) {
document.body.innerHTML+="<div id='iframe' name='iframe' style='display:visible;' ><iframe id='prova' name='prova' style='display:visible;position:absolute; top: 20px; right: 350px;' src='window.html' height='520' width='350' FRAMEBORDER='0' SCROLLING='NO'></div>";
} else {
top.frames["iframe"].style.display="";
}
}

dovrebbe andare


Ciao Matteo