if (Page.IsPostBack == false)
string other = WindowsIdentity.GetCurrent().Name.ToLower();
Where is that code running? Whose credentials are used?
Actually that's a really basic issue of web applications:
you have to differentiate between things happening in the browser (client), and thing happening on the server (IIS).
IIS is running with a local account of the server machine, and consequently your code fails.