You can direct download the video in AVI format here (right click -> Save target as...) or from MediaFire.
The song is Adayume (あだゆめ) from Suga Shikao's album SMILE.
They should release a compilation album with all Suga Shikao songs used in XXXHOLiC xD This is the sixth Suga Shikao song used in XXXHOLiC, I believe. There really is no better singer for XXXHOLiC =)
I really liked this OP, it's one of my favorites (if not the one), very mysterious and exotique.
Also available on YouTube: