The Windows XP and Windows 7 Gratuitous ARPs

Windows XP

Windows 7

ARP Announcement is used to refer to an ARP Request packet, broadcast on the local link, with both the sender and target IP address fields contain the IP address being announced.
Excerpted from RFC 5227:

   In this document, the term 'ARP Announcement' is used to refer to an
   ARP Request packet, broadcast on the local link, identical to the ARP
   Probe described above, except that both the sender and target IP
   address fields contain the IP address being announced.  It conveys a
   stronger statement than an ARP Probe, namely, "This is the address I
   am now using."

ARP Probe, is a new term defined in RFC 5227, released on July 2008.
Excerpted from RFC 5227:

   In this document, the term 'ARP Probe' is used to refer to an ARP
   Request packet, broadcast on the local link, with an all-zero 'sender
   IP address'.  The 'sender hardware address' MUST contain the hardware
   address of the interface sending the packet.  The 'sender IP address'
   field MUST be set to all zeroes, to avoid polluting ARP caches in
   other hosts on the same link in the case where the address turns out
   to be already in use by another host.  The 'target hardware address'
   field is ignored and SHOULD be set to all zeroes.  The 'target IP
   address' field MUST be set to the address being probed.  An ARP Probe
   conveys both a question ("Is anyone using this address?") and an
   implied statement ("This is the address I hope to use.").