Windows has gotten somewhat "chattier" over the years. What information is it communicating? Who knows?
I strongly suspect most of it is just "pings" or other ISP-control junk. The rest is probably a mishmash of software checking for updates. (Messenger, Winamp, Real, Norton Antivirus, etc.) I don't think it's sneaky-shady personal-info stuff, because the EFF would have told us already.
As long as your idle packets aren't too crazy-large, then you're probably A-OK.
If you're really curious, pick up a packet sniffer and point it at the same system.
- David Stein