If you are running both BizTalk BRE and Windows Defender, you may want to note that the BizTalk 2006 R2 documentation at MSDN has been updated recently to take into account a recent interaction with Windows Defender. The issue with Windows Defender was corrected immediately, and the fix for affected machines is simply adding localhost back to the hosts file.
This is the 2006 R2 documentation, but I believe the issue could also affect 2004, 2006, and 2009 Beta.
Posted by Karl W. Reinsch 