I can now capture audio/video from the Motorola 6412 to my PC via Firewire (my Firewire chipset supports Firewire 800 but I use the 400 port).
The problem was the Firewire drivers in XP SP2 and Microsoft's lack of making correct drivers for chipsets with Firewire 800 prior to the release of XP SP2. Using the Firewire drivers from XP SP1 fixes this.
The following was the solution for me:
1) Boot in safe mode
2) Rename SP2.cab in C:\\WINDOWS\\Driver Cache\\i386\\SP2.cab to SP2old.cab (this prevents XP from RECOPYING the following files which are replaced)
3) Overwrite these files in C:\\WINDOWS\\system32\\drivers
With older XP SP1 versions (which you can download off the net somewhere):
Note: If not done in safe mode and cab file not renamed, the SP2 drivers are copied back by XP.
There is a well known issue with XP SP2 and 1394 which may only affect those of us with Firewire 800. Mine is a Asus motherboard with firewire 400/800. Microsoft for whatever reason did not release compatible Firewire 800 drivers with SP2 which limits Firewire transfers to 100Mbps and can not allow capturing over firewire.
This article notes this:http://www.theinquirer.net/?article=17794
I came to the conclusion that the Firewire drivers were the issue when I tried to capture DV from a camera via Firewire for the first time in months, post SP2 install, and it failed (when it worked fine a few months ago prior to SP2). DV captured failed. I researched this. I fixed this and of course it fixed capturing from other firewire sources i.e. the Motorola 6412.
There is also a patch from Microsoft on their site somewhere for this that was posted in Dec 2004. This is a manual update. To be honest, it could have been the manual driver swap or this update as I did both on the same reboot prior to success.
UPDATE -- This is the Microsoft patch I used:
"Performance of 1394 devices may decrease after you install Windows XP Service Pack 2"http://support.microsoft.com/kb/885222/en-us
Here is more info on swapping the drivers and about the patch:http://www.rme-audio.com/english/techinfo/fw800sp2.htm
This patch and/or the driver swap back to SP1 1394 drivers work for me.
I really hope this helps someone. My only explanation why my XP SP2 did not work while others did, is that I believe I have Firewire 800 and I am guessing - those with working XP SP2 capturing setups use Firewire 400... or perhaps its something else. But this DID work for me. Note I do not mean I'm using a Firewire 800 cable from the 6412 to PC, its just my chipset supports both.
FYI, I am currently playing back using BSPlayer and the Moonlight-Elecard MPEG2 decoder and it looks great.