<FORM NAME="orderform">
What kind/s of music do you listen to?<BR>
<INPUT TYPE="CHECKBOX" NAME="music" VALUE="Rock" CHECKED>
Rock<BR>
<INPUT TYPE="CHECKBOX" NAME="music" VALUE="Reggae">
Reggae<BR>
<INPUT TYPE="CHECKBOX" NAME="music" VALUE="Pop">
Pop<BR>
<INPUT TYPE="CHECKBOX" NAME="music" VALUE="Rap">
Rap<BR>
<INPUT TYPE="CHECKBOX" NAME="music" VALUE="Metal">
Metal<BR>
<INPUT TYPE="SUBMIT" onclick="get_check_value()">
</FORM>

<SCRIPT LANGUAGE="JAVASCRIPT">
<!--

function get_check_value()
{
var c_value = "";
for (var i=0; i < document.orderform.music.length; i++)
   {
   if (document.orderform.music[i].checked)
      {
      c_value = c_value + document.orderform.music[i].value + "\n";
      }
   }
}

//-->
</SCRIPT>

We first initialize a variable c_value that will hold the values of the selected checkboxes. document.orderform.music.length determines the number of checkboxes with the name music. In our case, we can substitute this with 5, since we already know the number. The for loop iterates through all the checkboxes in that set and determines which of these is checked using an If statement. Once a selected checkbox is found, its value is appended to the c_value variable.
This code should be placed inside the HTML Head section (unless you know what you are doing).