Many manufacturers aren't releasing specific new drivers for very old components for the newest operating systems. They can't be expected to support things for the entire possibly lifetime of the hardware. The same goes for the video encoder chip on the card; the people that make that chip can't be expected to make new drivers for it forever, and Asus is dependent on them to get WinXP compatible drivers.
WinXP will have generic drivers for any TNT video card which will work for the normal graphics functions. The video in/out functions are not directly controlled by anything special on the card itself. They're simply a separate chipset built onto the board. If supported with built-in drivers, WinXP will detect the chipset at the same time that it installs the graphics driver for the TNT chip.
The v3400 seems to use a Chrontel 7003 video encoder chip. WinXP's hardware compatibility list doesn't have that chip listed, and in fact doesn't specifically list the v3400. So it's unlikely WinXP will recognize the video encoder as anything other than something like "PCI Multimedia Device" which it can't find drivers for, and the graphics will simply be using a generic TNT driver.
In short, the graphics will work fine as a generic TNT card, but the video functions won't work with WinXP.
I found this thread. It seems to indicate maybe the WDM 1.11 drivers will allow the chip to work, but it's not really clear if it's actually working for those people with the 3400, or only the ones with newer cards.
This one also seems to indicate that there are some workarounds that might make it work. A lot of reading to sort through all the people talking about their various models of cards.