This seems like it is a little late in the game to implement this, but the AdRotator control now supports an AdType property. This property can be set to popup or popunder. So now, you can use a builtin control to make your site as annoying as possible. Ironically, they add this feature, after they intergate a popup blocker into Internet Explorer. Just specify the PopPositionTop and PopPositionLeft properties to specify where the popup will appear.
<asp:AdRotator ID="AdRotator1" Runat="server"
DataSourceID="Ads" AdType=Popup PopFrequency="50"
PopPositionLeft="100" PopPositionTop="100"
/>
<asp:XmlDataSource ID="Ads"
Runat="server"
DataFile="~\Data\AdvertisementList.ads">
</asp:XmlDataSource>
The control now also supports ASP.NET 2.0 databinding. Therefore you are no longer constrained to using an XML file, you can use any datasource as long as it returns data in the required schema used by the AdRotator control.
Read the complete post at http://www.dotnettipoftheday.com/blog.aspx?id=185