Paul_o_b - 9:25 am on Aug 13, 2011 (gmt 0)
If you don't have a full doctype on your page then all versions of IE party like its 1999 :) and behave much like IE5. That means they don't understand anything new (and as such ie6 and under do not understand position:fixed in any mode) and in quirks mode (no doctype) then no version of IE will understand position:fixed (among many many other things).
Make sure you have a full and valid doctype and the fixed position will work in ie7+.
Use this doctype:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">