Probably automatic updates from something (Windows isn't the only place where automatic updates are possible; Adobe Reader, Firefox, Opera, iTunes, and many other applications do this).
If you have music sharing software or some bittorrent transfers active, then these can eat up a lot of bandwidth when least suspected as well.
Also, since you're using a Sony computer, you may have their highly controversial root-kit running. If this is the case, then Sony (or someone else who knows how to exploit it) may be using the root-kit to audit all your MP3s to determine if you're pirating music (I'm not sure how they differentiate between a pirated MP3 or a backup of a legitimately purchased original CD that was saved in MP3 format, but they have advanced algorithms for this according to the guy at their 1-800 number I called about this back when their root-kit was new) -- at least that's my understanding of the purpose of the Sony root-kit (to catch MP3 pirates).
There could be other SpyWare or viruses as well. These two free tools are excellent, and I believe they can detect and remove Sony's root-kit too:
SpyBot - Search & Destroy