error Insufficient permissions for setting the configuration section 'defautlProxy'

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C#

Internet

administrator

proxy

Hi,
In my windows form project, I used IWebProxy to config proxy credential, like as this solution that helps me:
http://stackoverflow.com/questions/186800/is-it-possible-to-specify-proxy-credentials-in-your-web-config[^]

but in a XP system I have this error (when the Application wants to connect internet with proxy):

Insufficient permissions for setting the configuration section 'defautlProxy'

Although that is an administrator user in domain!

how do I solve this problem.