I am using an ntl:home (virgin) cable modem, and even though I am not downloading anything the lights for downloading and uploading are on all the time. This only happens when I am downloading something like a torrent, or something from xbox live, but now even when I am not browsing the internet they still stay on, making my internet run considerably slower. Ive disconnected my wireless router as well to make sure that a neighbour wasnt using my internet, but the lights on the modem still stay on. Also, ive carried out a comprehensive scan using norton 360 to make sure there arent any viruses or spyware etc downloading stuff, and even after norton eliminated all the 'risks' it found, the lights on the modem remained on.

Does anybody have any idea what is going on here?

If you don't shut down your torrent provider properly it stays on and will upload downloaded files to other torrent members. So you are probably not downloading files but are uploading them.

Thats true. Regardless of whether you're uploading or downloading, the Data transfer light will remain on. shut down your Torrent client and have a look. The data transfer light should start to flicker and remain off for longer periods of time, only lighting up when your AV searches for updates.

Hope this helps