Problem in internet connection

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Mobile

Hi to all .i am using nokia 6300. I connect internet to my computer through mobile gprs connection.there are two problems , one is, sometimes the connection is automatically disconnected and displays a message "the modem is used by another application". But there is no other applications using the modem . Many times i try to reconnect.but it was failed.and

€ the second one is,my mobile (modem) is connected to my system.and it also display the speed of the internet and other details in the taskbar icon.but when i am trying to browse in the google chrome it shows "web server could not be found".that is there is no connection.and the task bar color also changed.i think this is a virus infection.but my anti virus did not detect anything. I am using avast 5.0.462.
How can i solve these problems? Please help me.