If you keep a watch on the "activity" LED on your broadband router or cable modem, you'll notice that your PC seems to be continually sending or receiving data via the internet, even when you're not actively doing anything that might lead to such behaviour.
So what's generating all that traffic? Viruses? Spyware? Someone hacking into your PC? Or just perfectly legitimate activity by programs that you've installed?
To find out, you need to monitor the TCP/IP traffic that flows around your network or your computer. Free programs such as WireShark are available to let you do this, but they're fiddly to use and you need a fair amount of technical knowledge to make sense of the results.
A simpler solution, and a great starting point if you want to know what's eating all your internet bandwidth, is a free utility called TCPview, from the SysInternals team at Microsoft. You can get it from http://technet.microsoft.com/en-us/sysinternals/bb897437 and it runs under all versions of Windows from XP onwards. The program was updated to version 3.04 a couple of months ago.
TCPView is only a 0.3 MB download, and it doesn't need any installation. Just download the .zip file, extract it, and run the application file. I recommend that you then go to the Options menu and turn off the "show unconnected endpoints" feature. You'll be left with a constantly-updated list of which programs are communicating over the internet from your PC, which will help you track down any suspicious activity.