mpersonation in ASP.NET (IIS 6.0)

i have enabled impersonation in my asp.net application. it works fine in my local machine. i have published the site onwindows server. in my code i will generate an excel file and save it on a folder there in server . when that code executes the prompt for servers username and password appears. when i enter the user name and password it works properly. But since i have enabled impersonation in my web config i would n't be asked for username and password. how can i avoid the window that is asking username and password when my code to save a file on server .......??? ...