Image Map Blocking ClientX In IE?

I'm doing some code to show an image where a user clicks on an image map. I've got code that tracks the mouse location and uses that to place the image where you click (it also stores the value in some inputs so the co-ords are sent through post).

Works perfectly fine in FF but in IE it stops tracking the mouse's location once you move over the image (so when you click it places the image at the edge).It should be tracking the client mouse at a document level so I find it bizarre it doesn't track it for this one element.

Code:

<script>
if (window.Event) {
document.captureEvents(Event.MOUSEMOVE);

[code].....