"The x64-based versions of Microsoft Windows Server 2003 and Microsoft Windows XP Professional x64 Edition include a 32-bit version and a 64-bit version of Microsoft Internet Explorer. Both versions of Internet Explorer are included to increase compatibility with existing programs and Web sites. The 32-bit version of Internet Explorer can host only native 32-bit ActiveX controls and other 32-bit Web page objects. The 64-bit version of Internet Explorer can host only native 64-bit ActiveX controls and other 64-bit Web page objects.
This article describes the differences between the 32-bit and 64-bit versions of Internet Explorer that are included in the x64-based versions of Windows."