Oh sorry, I was not trying to imply you didn't know lol just offering a suggestion. Also since your reply I went back to reread what you were asking, for some reason I didn't read all your original post. I am not sure how I misread what you were looking for, I didn't see how the user was logging off and that you were asking for checking inactivity of the browser
So if you cannot use anything that relates to asp.net why don't you persuade them to make it to winforms network related?It's like asking you to make a win driver without using a low level language(well sort of but you get the idea).