On my system the device which is decoding Mpeg stream is graphic unit, the sound is being decoded by sound card again on hardware level. CPU handles only renderer, but it is not strong enough (Pentium III-s, 1400@1638). Pentium 4 does not perform much better.
MPEG (I assume you are referring to MPEG 1 or 2) is not the same thing as HD (H264, the most common codec used). Many old graphic cards have hardware MPEG acceleration but this is useless for HD.
Sound in HD video is usually AC3. I don't know about any sound card that can decode AC3. Passthrough to an external receiver - yes; hardware acceleration for AC3 - never heard of it. Most of sound cards can't even decode MP3...
CPU handles only renderer?? You contradict yourself. If video decoding is done by video accelerator, then rendering is too. Video stream cannot go to graphic card for decoding, then return to CPU for rendering (?!) and then go back to graphic card for display on screen. In the last 15 years rendering has been done by hardware acceleration: overlay or DirectX VA.
There are two other operations that must be done by the CPU: reading the file and splitting it into video and audio streams.
If you need hardware acceleration for HD, first you need a new graphic card that can do that. I don't know any that still have drivers for Win98.
I have ATI Radeon 9800 Pro. It can decode MPEG2 in hardware, but not in Win98 because the codec (Nvidia PureVideo Decoder) doesn't work in Win98.
Hardware decoding in Win98 is a problem, not only for HD, but for MPEG2 too.
You said that the container is WMV? Video in HD WMV files is usually Windows Media Video 9. I don't know any card or codec other than M$ that can decode WMV9 (hardware or not).
Edited by Marius '95, 01 November 2009 - 03:04 PM.