My modem was installed in the last week of Dec 07. But the password was given only on 02.02.2008. They just gave me the password even without my signature on any document.
check your detailed usage to see if the downloading/uploading is being done in the last week of dec.also check if your IP address is something like 117.x.x.x.also do you use
windows dialup(eg. connect to bsnl window in which your username is there & also password if you tick the remember my pasword option) or you can simply browse the net without using your username/password everytime you connect by just switching on the modem & do not using windows "connect to" option.
there are two ways to setup a modem to connect-----1.pppoe,2.bridge
in pppoe your username & password is stored inside modem usually by the bsnl person who come to install.in this once username/password is saved you just need to turn on the modem to connect.
in bridge mode you use windows "connect to bsnl" option by creating a connection using new connection wizard.in this when you double click on the icon or choose connect to from start menu a window come up asking for username/password.
all the IP having range 117.x.x.x are port binded which means your username/password can only be used from your telephone line & nowhere else.
if you have an IP something like 117.x.x.x & you are using pppoe method & all your usage is from the date your modem is installed then it may be possible that the bsnl person setup your modem in pppoe mode & from that time anyone can connect to internet when modem was on & you didn't even realize that.