In a windows network I'd like to set up some kind of audit, to know when certain users access certain resources (e.g. copy files)
Is there a simple way to have such operations logged?
To log incomming and outgoing connections from a network, you may want to use wireshark.
Windows comes with a built-in logging suite. In windows 7 simply type logs into the start->search bar
Process Monitor, set a filter such that Path must begin with the path your files are in.
This way, you can double click any event to see the user that caused that event...
If needed, you can also filter by certain file actions like writes and deletes if reading is just fine.
PS: Make sure the Drop Filtered Events option is enabled, you don't want to fill your memory/disk. :)