Task Manager shows network activity even when idle

I access internet through Realtek RTL8139/810x Family Fast Ethernet NIC LAN
ethernet card. When I log on my ISP account, and surf (or download), there
is obviously network activity when i check task bar 'Network Utilization".
Since past few days, however, i see certain amount of bytes being 'received'
even when i log off from ISP account and stop surfing. And this byte amount
is not little, but is constant and substantial. I wonder where are these
bytes coming from and getting stored on my PC. Can anyone help me
out with this?

I tried to block activity at various ports using Norton Internet Security
Pro 2003 firewall and inbuilt Win XP Internet Connection Firewall, but to no
avail. In fact, when i checked the ICF, I saw Windows Messenger (msmsgs.exe)
creating some nuisance here in Services tab. It automatically enables
internet user access on many ports for TCP and UDP. I
disable them, they get enabled automatically next time i check. My NIS 2003
firewall tells me msmsgs.exe accesses port 22 (telnet and SSH port) even
when i terminate it from Task Manager Services tab. It restarts itself gain
and again and I find no other way to disable it. I want to disable it coz I
dont use Windows messenger for IM.

I will be checking this thread for any replies. If convenient, leave contact
instructions in case i wish to ask couple of more questions. Help will be
appreciative.