help needed to modify a URL parameter

My Javasript program_1 receives URL and parameter
(i.e. http://mysite.html?xxx )

program_1 then goes to next page using window.location

If the user presses the browser 'back' key, then I want the
URL and paramater to be http://mysite.html?zzzz
so that I can detect that they have used the browser 'back' key
and take other action.

Alternatively, is there any way to detect / identify that the user has used the 'back key' in Internet Explorer ? Program_1 supplies purchaser with an access code, and I do not want the user to be able to get a second code.

Any suggestion how to do this?

you can not change the url without adding the page to the history unless you play with hash.

Eric

thanks - However, do you know if there is any way to be able to detect that the user of Internet Explorer has used the 'back key'?