Upload is more than download

Hi,
I am using netgear router model CVG834G. I am able to login to default gateway of my router (http://192.168.0.1) using User Id:admin and Password: password.But i am unable to change the password of my router's default gateway.Can anyone please help me to change the password?
My wireless connection has limit upto 30GB consumption. But last couple of months i noticed that upload is happening for 60GB and download is as expected.
I am not using an P2P software also.
Changed my broadband SSID . Stop broadcasting SSID. Am using WPA2-PSK encryption mechanism to protect my network from intruder.
In spite of all this pre-caution there are 60 GB upload at the end of month.
Can anyone please suggest to find out how this abonormal upload is happening?
Who is doing this upload so that i can block those IP address?
Thanks in advance.
BRs
dharaprd