>> What's the main difference between using standard MS standards like ASP, COM object, SQL Server, etc. to build application thru the Web and using XML?

There is no diffence, since it's really apples vs. oranges. You can use ASP to return XML, use COM to build an XML string, or soon use SQL Server to return XML, update from XML, or store XML. Some products might just use XML to store files, others will use XML internally to pass data around, etc. The thing to realize is that XML has nothing to do with the web. But, it will also change the web more than any other technology in the past, including HTML. In fact, HTML will be created from the transformation of XML via XSL, either on the client on the server. XML will turn the web into a WWD (world wide database).