Display Html Depending On Selected Items In Listbox

i'm after some script to display some html depending on what is selected in a list box
the html i want to display are checkboxes so it's more stuff for a form i'm using lotus notes, so i have to use javascript.

i can get it to display a checkbox if a particular item is selected but it won't show up in place, it loads a new page with the check box the only thing on that new page.

function updateChecks()
{
string = "";
if (document._WEBRequest.dataReq.value == "blah") {
string = "<input type="checkbox" name="check" />";
string += "Boundary";
}
return string;
}
function writeChecks()
{
document.write(updateChecks());
}
the writeChecks() function is called onChange for the listbox

also don't have much clue as to how to make it show more checkboxes if more than one item is selected in the listbox.