unknown internet activity

Hello. I performed a clean installation of windows 2000 professional on ntfs hard drive. the first thing i did after installation was to download service pack 4 from the windows update site and all the critical updates. when i connect to the internet. may computer starts sending data continuously and i do not know where or why! I do not do anything and over an hour it sends about 30MB and receives about 3MB! I cannot explain this activity. Can you please help me? Thanks in advance