There are a lot of things, you said "at work" and your wording suggests multiple users - ie. you are not the only one at that office.
1. Someone has a virus and it is broadcasting - Broadcasts have to be processed by your CPU, which is why your machine sees packets. - hence the network activity on your network card
2. You have a file/print server, you have a document/program open on the file server and there are network keep alive messages holding a drive mapping or connection open for this document/program - hence the network activity on your network card
3. You have an email server, your client will need to continually check with that server to see if you have mail - hence the network activity on your network card
I could go on and on, but the bottom line is that if there are just a handful of machines on a network, there will always be network traffic, keep alive messages, MSBrowser announcements, DHCP messages etc etc etc