in all reality, HTML is xml - that just uses a more standardized doctype. You can not have a straight up .xml file display as html - but you can have a server-side language parse your xml and display it as html
Sorry - correction - I think IIS allows you to display .xxx as .yyy filetypes. So I think you can force any document type display as anything you want (if you have access to iis that is). You can probably do the same in Apache as well