Could not find file of path

Hi All,
I seem to be having problem when attaching files with my email.  I have a page in my website that allows users to upload files to attach with emails.  It seems like whenever the application calls this line of code:
message.Attachments.Add(new Attachment(filename));
the process fails and returns this error:
"Could not find file of path "C:\Documents and Settings\user\My Documents\file.txt"
which definitely exists! I am assuming that the application is trying to locate the file in the server in which case I am at lost!  How can I tell it so that it points to the user's local drive?  Do I have to configure something in my web.config.  By the way, identity impersonate is already activated in my web.config.
Thanks All!
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