IE and CSS issue?

I swear I posted this before, but I can’t find it. Sorry if it’s floating in the forums somewhere.
Anyway, I have a background image on my blog (www.elmsblog.com) and it is not supposed to move when the user scrolls. I have valid CSS and everything.
It works as it’s supposed to on my Mac with IE, Firefox and Safari. On my dad’s window’s machine, the pic SCROLLS! He’s using IE, but I don’t know what version. Could it just be outdated? Is this a bigger issue? I haven’t gotten to try with any other browser on Windoze.
Thanks.