This is caused by people referencing graphics on the MSN site from here (the little icons by names, etc.)

I took the homepage, which is currently causing that irritating popup, and narrowed it down. Adding this link to a page (no offense to the person who owns the graphic) will cause the popup to open:

<IMG HEIGHT=10 WIDTH=10 SRC="http://www.msnusers.com/ffnk5ft4rkabah2vgjavushl36/Documents/Pictures%2FFort%20Polk%20Pics%20009.jpg" BORDER=0>

I have no idea how to stop it unfortunately.