I use windows XP sp3 on an HP laptop compaq nc6400 connecting to the net through adsl connection in a local network, as soon as I connect to the internet it starts to receive packets, is a virus doing this or it's related to settings what can be the reason? please help me, thanks. If any extra information needed pls let me know.
Safest way would be to download wireshark and check it out yourself. You'll be able to see the source and destination of packets.
It does not have to be a virus. Many programs (including Windows itself) have automatic update features which will connect to Internet and check if there are updates available.
Also Windows uses Internet connection to check if your computer's time is correct.
There may be few other background asks I may have missed.
Microsoft's freeware Network Monitor is also quite useful. It's not quite as hardcore as Wireshark (which I also use) so it's perhaps a bit easier to use. I particularly like the way it organizes results and associates traffic with specific applications and services.
Keep in mind that there will always be some background traffic related to network maintenance.