Try removing youyour original web.configweb.config (create a backup first), and after making changes via IIS (it will create a new web.configweb.config), restore the original changes you'd made to the file. In case you are configure HTTP ERRORs follow this http://paymentgatewayintegrationhelp.com/Tech-Help/HTTP-Errors-and-webconfig-Lock-Violation-1140.asp
Remove mildly sketchy link, as it no longer works and everything it used to cover (checked via web.archive.org) is touched on in other answers