When I link down my computer, there are still download and upload traffic to internet. What is wrong and what can I do to fix it.
You mean that your internet connection is always active?
If that so, then there're a few reasons: Some services/programs are updating or they are transmitting some data in background. However, it could be anything.
Just in case, do you have some signs of malware: http://www.virusvault.us/signs1.html? It's possible that malware is doing this problem.
If you seem to have infection, read the thread "BEFORE you POST"(READ this Procedure BEFORE Requesting Assistance) (http://forums.spybot.info/showthread.php?t=288) and by following the steps from thread, create new one in the Malware Removal Forum (http://forums.spybot.info/forumdisplay.php?f=22). Write your problem so that assists could know what to do.