Heya... been lurking around this post since I got a 3850 back in August (and it still don't work right
), but I'm much better off with the info I've found here. Thought I'd attempt to give a little back.
With regards to DXVA... you just need a decoder that supports it. To my knowledge, there are only 2 at the moment:
1) Cyberlink's PowerDVD Codecs, which actually are absolute junk, especially since they expect you to pay a lot for them, and they don't even work right (vids play smooth, but still at 100% CPU).
2) Or Media Player Classic - HomeCinema edition. This is what I use, and it definitely has some quirks, but it works well as long as your video is encoded to conform to DXVA (which a LOT of H264 content packed in MKVs on the net are not, but people are getting better about this lately). I haven't had any problems at all with VC-1 content.
Get it here:
http://tibrium.neuf.fr/DXVASupport.html
http://sourceforge.net/project/showfiles.php?group_id=170561
Set it up in Playback-->Output to use VMR9 (renderless) in XP, or EVR in Vista with .NET 3.0+ Installed. Set Surface to 3D surfaces, and resizer to Bicubic A=1.00. Under Internal Filters, make sure everything is checked (you want to use the built in splitters/codecs ONLY!). If you have ffdshow or any other third-party codecs set to auto-load, you'll need to either disable them or lower their merit (with something like RadLight filter manager)
You can confirm DXVA is working by right clicking on the video once it's playing, and under Filters, you should see "MPC Video Decoder". Select it, and on the first property page you see it'll say whether DXVA is active or not. Or...just look at your CPU use. I run an old Athlon XP Mobile, but it still doesn't go above 5-10% CPU when it's active. (Otherwise, HD content is completely unplayable for me)
There's a newer version of the player packed with CCCP (google), but they disable the hardware decoders by default for some odd reason.
Hope that helps...