Popup confirm not working.

Because you are telling the code to change location to ArchiveContactandCases.asp every time.

You do this nClick="GP_popupConfirmMsg('Please confirm you wish to archive this contact.\r\rNote that all of this contact\'s cases will also be archived automatically.');self.location='ArchiveContactandCases.asp?NewUserid=<%=(ContactDetails.Fields.Item("UserId").Value)%>';return document.MM_returnValue

which is like this:

GP_popupConfirmMsg('Please confirm you wish to archive this contact.\r\rNote that all of this contact\'s cases will also be archived automatically.');
self.location='ArchiveContactandCases.asp?NewUserid=<%=(ContactDetails.Fields.Item("UserId").Value)%>';
return document.MM_returnValue

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As you can see here, it doesn't matter what happens in the GP_popupConfirmMsg, you always send the webpage to the Archivecontact... page in the very next line.

From google, I think the function you are using is like this:

function GP_popupConfirmMsg(msg) {
    if ( confirm(msg) ) {
        window.location = 'my page blah.asp';
    }
}

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It is in this function that you should put the destination... (and any other code that should happen based on the selection.

It uses a dreamweaver extension, i add the first one to confirm, then add the next one to redirect IF first one works.
Is there a way to stop on the first one if user clicks 'cancel' ?

No solution was added ... closing this question.