Well, as a general way of fact, if you ask for help about an error in a certain line of a file, the minimum you can do is to explicit what is said in that line -this is what I said this morning.
This being said, perhaps I answered somewhat too quickly (even if I arrived late). The web.config is somewhere in "C:\Documents and Settings", this is what appeared in the errors window.
Anyway there seems to be a major error in what you say : Nikhi's WebDevHelper is not supposed to be installed on the server, but on the client machine. No trace of it must appear in web.config.
Nikhi's WebDevHelper helps to explicit the exchanges that IE did with the server. This helps you to verify the impact of an UpdatePanel, for instance, where only a part of the page is refreshed.
I was looking for it for I could not make it appear in IE8 : I was looking in the Tools menu, but it seems you have to look in the Display menu, with the exploration panels.