Software for Internet trffic monitoring on my Pc

Is there any Software by which i can understand the incomming and outgoing traffic on my computer.

My modem Sent/recieve lights are contiously on butr the speed is very very low.

I think there is application in background which keeps on sucking my internet bandwidht

Is any software is there which tell what is upstram and downstream traffic
and which application is sending and receving data from internet.

Freeware is most proffered

Thanks in advance