Trouble sending UDP datagrams on Win2008

Hello all -

I have an application which loops through a wide range of sequential IP
addresses, sending a small UDP datagram to each one. The app does not
check whether a computer exists at the IP address first (tried this
before by pinging -- way too slow). This works fine on Win2000 and
Win2003 -- nice and quick. On Win2008, however, it is very slow. By
using Network Monitor, I see that Win2008 is sending three ARP requests
to each IP address at which no computer exists, and won't let the
application continue until it finishes the three ARP requests (this
takes about one second per IP address). On Win2000/Win2003, only one ARP
request is sent per IP address, and control is returned to the app
instantly.

The application uses standard Winsock API calls for sending the datagrams.

I would appreciate any suggestions for speeding this thing up on Win2008.