This is a great post:
http://www.adopenstatic.com/cs/blogs/ken/archive/2006/09/30/Certificate-Services-Website-generates-404-on-Windows-Server-2003-x64-_2800_64-bit-edition_2900_.aspx?CommentPosted=true#commentmessage

The post is in reference to Certificate Services web site, but pertains to any situation in which an IIS application expects to be running in 32-bit mode, but IIS is actually running in 64-bit mode. I had this issue with the VMWare Server management web site.