Networking & wireless forum: LAN SPEED (Windows-to-Mac=Fast/Mac-to-Mac=Slow)

I've scoured the Web and this board for a solution to this problem and as of yet have come up with nothing.

THE PROBLEM:

For some odd reason which I am unable to resolve through my own exhaustive efforts, I can copy across the LAN a large file like a 1G-to-2G movie file from a Windows machine to either of two Macs in a zippy 1-minute or so. If I try to copy the same exact file from one Mac to another Mac, it takes upwards of 30-minutes.

In this particular case all of the computers are ''hardwired'' via Gig-Ethernet to a D-Link DIR-655 router.

Now, the kicker is that this isn't consistent. For a while this problem had disappeared and I was getting equally fast speeds all around the LAN i the minute-to-two-minutes range. Even going wireless-N took only 2-to-3 minutes max.

THE NETWORK:

I have a typical small network. On this network I have two Macs and one Windows machine.

The Macs include an older Power Mac G5 (2004) running the latest updated Leopard 10.5.8 and a brand new MacBook Pro i7 (2010) running the latest updated Snow Leopard 10.6.4.

The Windows machine is a Dell XPS 410 running the latest updated Windows XP/SP3.

The network router is a D-Link DIR-655 and my ISP is Verizon DSL via a Westell WireSpeed 6100 Dual Connect modem.

Why are Macs ''arms open wide'' to a Windows machine but not to it's own kin?