What music track was used for the "Ring of Fire" (Emile Griffith
Story) promo from usa networks?

http://www.usanetwork.com/movies/ringoffire/video/player.php?id=rof_redemption

The music in the promo for "Ring of Fire" is a snippet from Clint
Mansell's "Lux Aeterna," performed by the Kronos Quartet. This music
was on the soundtrack of the film "Requiem for a Dream." It was also
used in a trailer for "The Lord of the Rings: The Two Towers."

"When the commercial for this movie comes on USA, what is the song
playing in the background? It sounds familiar and it has been killing
me for days. It sounds like violins. I think I've heard it from
another movie's soundtrack, possibly a horror or thriller. Any
ideas???...

It's called "Lux Aeterna"... originally from the movie Requiem for a Dream."  

Internet Movie Database Messageboards: Ring of Fire
http://www.imdb.com/title/tt0436720/board/nest/17472868

"Lux Aeterna" is on the "Requiem for Dream" soundtrack CD:

Buy.com: Requiem for Dream Soundtrack
http://www.buy.com/retail/product.asp?sku=60435241&loc=109&sp=1

You can listen to a remix here:

Requiem for a Dream
http://205.166.53.100/Audio/Dream.mp3

I hope this is helpful. If anything is unclear or incomplete, or if a
link doesn't work for you, please request clarification; I'll be glad
to offer further assistance before you rate my answer.

Best regards,
pinkfreud