well, first of all .aspx and .html pages are way different in their processing. so you can either have an "html" page or an "aspx" page load at any given time. However, if you want just the URL to be changed to html but still process the aspx page internally, then you need to rewrite the URL. Following are some links that tell you how to do URL rewriting in ASP.NET running on IIS.
URL Rewrite in ASP.NET[
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URL Rewriting with ASP.NET[
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Hope this helps. Oh and also if you have IIS 7 then you can use something called URL Rewrite within IIS to setup rewriting rules that essentially just change the URL syntactically from anything to anything but still run the right server side code.
For e.g.
http:
would become
http:
Hope this helps and hope this is what you wanted...
Cheers.