Have you ran netstat to see how many connections are open and what they are ?
Spyware is notorious for hogging bandwidth, so if you do have a lot of open ports to the internet, then run spybot search and destroy as this does a good job of clearing them.
http://www.safer-networking.org/en/index.html
If you do have spyware software already, how often do you run a full scan and update its definitions ?
I know that last week I had about 3 new updates alone for Spybot.