There's so many things it could be. I always have task manager on (XP) to see what processes are running and how much network is being used. Even if you are downloading at less than the speed you are at with your provider, it still doesn't necessarily mean it's a virus. It could be the host has a lot of traffic going, your computer is using too many apps, network usage from other devices could be slowing it down, uploads might be...
Check your connection speed here to see if it's at where it should be.
http://www.speakeasy.net/speedtest/
Again, if it's not as fast as it should be, it does NOT necessarily mean it's a virus. Just scan your computer for viruses weekly or more as necessary if you are visiting unfamiliar sites and downloading a lot of content.