Prova questo....
codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>Untitled</title>
<script language="JavaScript" type="text/javascript">
<!--
var pluto = false;
function pippo(aOggetto,lunghezzaMassima) {
if (pluto && parseInt( aOggetto.style.width) < lunghezzaMassima) {
aOggetto.style.width = parseInt( aOggetto.style.width) + 1;
} // if (pluto)
} // function pippo(aOggetto,lunghezzaMassima)
//-->
</script>
</head>
<body>
<div style="width:0px;height:20px;background-color:red" id="mydiv"></div>
<input type="button" value="++++" onmouseover="pluto=true" onmouseout="pluto=false">
<script language="JavaScript" type="text/javascript">
<!--
setInterval("pippo(document.getElementById('mydiv'),200)",50);
//-->
</script>
</body>
</html>