Microsoft has slipped in a new security 'feature' in Internet Explorer 8 which prevents .Net components from loading in Internet Exporer from sites in the Internet zone. Internet Explorer version 6 & 7 did not have this limitation so applications may break after Internet Explorer 8 is installed.
Here's the Microsoft Knowledgebase article that describes the change and how to work around it.
http://blogs.msdn.com/askie/archive/2009/05/22/net-control-no-longer-loads-in-ie8-in-internet-zone.aspx
