Dunno what version is supported by Windows XP, but I know there must be some because GPU decoding on MPC (Media Player Classic - http://mpc-hc.sourceforge.net/) works great on Windows XP.
VLC v1.1 supports DxVA in its version 2.0. That means that Vista, 2008 or 7 are required. If I am using Windows XP, GPU decoding in VLC cannot work for me. I would like to request this feature on Windows XP.