Some of the viruses and spyware that is going around today can be really tough to get rid of. Lots of the time you will have to run many tools to get rid of it all. There will be other times when you remove the infection and it comes right back or you cant get rid of it at all. Have you tried multiple virus/spyware programs? Check out our antivirus and spyware pages for more information.
You can also check your Programs and Features or startup items (MSconfig to find things that aren't supposed to be there and remove them. There may be something running in the background that is causing your modems extra activity. Its also better to do your virus and spyware scans in Safe Mode for better results. You can also find sites that offer free online virus and spyware scans.
Check out Windows Task Manager to see exactly what's running on your computer. You can look at the processes tab and check for anything out of place.