Hi Leo,
Are you running any virtuallization software (for example, VMware Workstation) on this machine? Is there any reason the NIC might be in promiscuous mode (for example, running a packet capture software)?
I've seen this issue before, but only when the NIC was in promiscuous mode (which some virtualization softwares will do when you put the virtual NIC into bridged mode).
I've only noticed this issue on Windows 7, though, so I'm not sure if you're experiencing what I've seen or not.
James