Try going back to 2.998 drivers.

Your guess about XP going 100 speed is a good start - check your SidSpeed value in your Registry (you'll find detailed instructions in this thread:  http://www.rme-audio.de/forum/viewtopic.php?id=4123 )

Have you checked your latency value for the Fireface Settings?  It could have been changed somehow, to a value that is too short for proper operation with your system. 

Have you done any upgrading/updating to your XP installation recently?  Perhaps try a System Restore...?

I know the Glyph Manager software can be buggy - try ending that process and seeing what happens (the drive will still work).  In the Glyph Manager program > Options, I disable all four check boxes, so that it doesn't run at startup, and doesn't linger in the taskbar.  You can still run it directly, to check on your drive's health, but otherwise, it seems to be better to quit it entirely.  (I'm assuming you're using a Glyph 062 or 062E RAID drive, which uses this application - otherwise, nevermind...)

Try disabling the Realtek audio driver in Device Manager during the times you're using the Fireface, to rule out that possible conflict.

Okay, if you can't find the SidSpeed entry in your Registry, using Regedit, then you need to create it.

First thing:  You need to look for the correct ID number for your Firewire controller in the Registry.

To know what's the correct ID go to:
Control Panel->System->Hardware->Device Manager
Find IEEE 1394 Bus Host Controllers - click on the "+" sign next to it to show the sub-entry:
"Texas Instruments OHCI Compliant IEEE 1394 HOst Controller"
Right click on that, and select "Properties"
In the "Details" tab you can see the Device Instance Id.
This is the ID number that you should look for in the registry.  Write it down now, or keep the Properties window open for now.

For example only, my ID is:
VEN_104C&DEV_8025&SUBSYS_00000000&REV_01

followed by a "\"

and then 4&6B16D5B&0&18F0

Yours may be different.  In any event, you'll want to use YOURS.

Click your Start button, click on "Run" and in the window that opens, type "regedit" (without the quotation marks).

Go to:  HKEY_LOCAL_MACHINE and click on the "+" sign to open its contents.
Go to:  SYSTEM - open its contents
Go to:  CurrentControlSet - open its contents
Go to:  Enum - open it
Go to:  PCI - open
Go to:  the ID number you found above, starting with "VEN....." and going to the first "\" - open it
Go to:  the number that followed the first "\" in the ID number you found above, like "4&6B..."

In other words, go to
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Enum\PCI\VEN_1-4C...\4&6B...

In the right side of the regedit window, you should see the contents of this last entry. 
At the very least, there should be something called "(Default)" and a second entry called  "SidSpeed".

Now, here's the crucial bit:  If there is no entry for SidSpeed, you can create it by right clicking anywhere in that right side of the Regedit window, and using the pop-up menu that appears, create a new item.  Name it SidSpeed - just like that, all one word with the two capital "S"s.

Once it's created, you need to enter its value.  Do this by right clicking on it, and select "Modify" from the pop-up menu.  A window will appear with a field for "Value data", and two radio buttons for selecting either "Hexadecimal" or "Decimal".  Select "Hexadecimal", and enter a "2" in the Value data field.  Click "OK" to close the widow.

Close/Quit Regedit.
Restart your computer.

If the SidSpeed setting was indeed the source of your problems, then your Fireface should start to function properly now.  If not, well, sorry about all that...

Now that you've created the SidSpeed entry, you can find it again easily by using the "Find" function in Regedit, found under the "Edit" menu item.

If you like, you can experiment with entering a "3" instead of a "2" in the Value data field mentioned above, as the value of SidSpeed.  This is supposed to enable speeds faster than Firewire 400 (which is to say, Firewire 800), but there is some debate as to whether this is actually necessary.  I've found both to work with my Fireface 800.

Just enabled internal soundcard and used the spdif input on the mb from an ad converter and everything works.

Hurry Dr. Watson, game is afoot!

And what are the errors being counted in the settings dialog?

FW643 (FW800, PCI ID 5901) Revision 6: connection problems, stop after playback starts - <= this one

Rev 6 is the culprit then. If it worked before then it was just luck. A minimal change in whatever hardware value/parameter and it enters the 'normal' mode - not working.

Cable and casing *should* be ok if it runs on OS X, but it's not for certain, because Windows and OS X handle Firewire slightly different as far as I understood (that's why I proposed using a pre 2.998 driver, because that makes a difference, too). But before trying anything else you should really try another cable and hub to make sure. Since it worked before you need to find out what *changed*, if it's not some hardware change (like broken cable) there must have been a software change (installed new firmware, new service pack, new bootcamp, new anything?).

You cannot upgrade to a rev. 7 chipset, it's soldered into your mainboard, but you can try a Firewire ExpressCard.

If you have a drive spare (you should!) then you could try to do a clean installation of Windows XP with just the bare minimum drivers for testing. If you find out that it works there you know it's some software problem, if it doesn't you know it's a more serious issue and likely a hardware problem.

So with the talk of ExpressCards and Windows here in this thread, I was wondering: is there some FW ExpressCard that is known to really work in Boot Camp, instead of produce the Code 12 error that seems to be very common with them?

Ah yes that makes sense. I guess more waiting to do, or using XP and not being able to access all the RAM. (I'm presuming XP x64 won't work either)