<p>I have all the web pages of a website. My task is to change all HTML files to <code>.asp</code> files and change the links. I have about 280 HTML files.<br>
Is there any software or web service which can read a website and show me the link structure of the site (to make my job easier) similar to a site map?</p>

If the links use relative paths, all you have to do is to change the .html file extensions to .asp and place the files on a server that can run ASP.
If they are absolute paths, you can get away with editing your hosts file to point the original domain to your own server and just place the untouched files on your server.
If you
really
need to replace the URLs, try a text editor like UltraEdit or Notepad++ (on Windows) that can do find/replace over files (and use regular expressions for the search phrase). Or you can use sed or Perl on Linux.