Three methods are available but you will have to abandon the iFrame unless you put it in a div according to the coding below. Anything fixed will mean other content will scroll behind it and be hidden.
Coding most of the page as position: relative with floats but the centre part as position: absolute and stated px for left and top will usually make it stay in one place while the rest scrolls.
Position: fixed is the best solution but IE6 does not support this. IE7 and Firefox do.
See also
http://tagsoup.com/-dev/null-/css/fixed/ (scroll and watch the left image)
http://www.exclipy.com/css/imageoverlay/ (image overlay which stays in fixed position - works in Firefox but I can't see the effect in IE).