Hi,
It could be a few things;
does the NIC have any WOL settings which might trigger it to wake up, or, does it have any Power Management settings.
Do you have any external USB drives, some of them have their own circuits which 'ping' the computer, to check that they're still attached.
Some programs, like CD/Media ones, routinely check the CD/DVD to see if there's anything in it.
Colin