// javascript function

<script type="text/javascript">
function switch_os(os) 
{
	document.getElementById("mac_field").style.display = (os == "mac" ? "block" : "none");
	document.getElementById("windows_field").style.display = (os == "windows" ? "block" : "none");
	document.getElementById("linux_field").style.display = (os == "linux" ? "block" : "none");
}
</script>

// html code to change OS

<select name="file_type" onChange="switch_os(this.value);"> 
<option value="" selected> Select OS...</option>
<option value="mac">Mac</option> 
<option value="windows">Windows</option> 
<option value="linux">Linux</option> 
</select>

// how to make this work with onMouseover of image/text ??