Impersonate doesn't work!!!

I am trying to save a file to a file share. In my
web.config file I added "<identity impersonate="true"
userName="domain\user" password="password" />." This work
fine in my development enviornment. However, when I moved
it to my production server I get the error below. Also, in my machine.config file I have the username under processModel equal "SYSTEM". It works fine on my development enviornment with username equals "machine"

Here's the error:
Parser Error Message: Could not create Windows user token
from the credentials specified in the config file. Error
from the operating system 'A required privilege is not
held by the client. '

Can someone help me?

Thanks

James