This is difficult to answer without knowing what you have installed on your machine. However, here are a few suggestions: I will skip the obvious about how you may have trojans or spyware etc...
Alot of software has an option to regularly check for updates including Windows itself. Likewise, some software also "calls home" so to speak, a prime example being Zonealarm. This type of thing will add to the traffic outgoing from your machine.
Also, as a possibly linked cause, you could check the services that are running on your machine. Go to Start - Run - type "services.msc" (without the commas and assuming you use XP). A good website explaining your services is at
http://www.blackviper.com/
Also check your task manager against the listings here
http://www.answersthatwork.com/Taskl...s/tasklist.htm
To see what is connecting to what, go to your command prompt and type "netstat -a", (again, without commas). This will give you a detailed listing of processes and what ports that are connecting from, both TCP and UDP packets.
If you go to
http://www.sysinternals.com, there are some cool freeware apps that can check whats what both inside and outgoing from your system. For example, they have a process explorer and a netstat app. with a nice fancy GUI. Hope this helps.