OK, the problem is the new HTML includes an XHTML doctype which causes IE 6 to behave differently. With that doctype it does not like two body style definitions in your css (at least as regards background-attachment working).
You need to remove BODY from this:
Code:
BODY,DIV,TH,TD,P,INPUT,SELECT,TEXTAREA,TT {
FONT-FAMILY: Verdana, Arial, Helvetica, Sans-serif;
COLOR: #2c3e49;
FONT-SIZE: 11px;
}
and copy the font-family, color and font-size down to your body definition with the background-image in it.