I've had some good results with ffdshow x64 (standalone)
If you're interested, you can obtain the files from Digital Digest at this link:
ffdshow -> History | Software - Digital Digest Note: scroll down to find x64 version.
I use this mainly to play XviD within Windows Media Center.
It would also be beneficial if you run the x64 version of WMP. I would post the instructions, but don't know if they've been already stated.