Musical Bleg

One of the most annoying things in the world to me is knowing a song by tune but not title, and not being able to find it. Normally, I'll just punch the lyrics in a search engine, but that doesn't work for instrumentals.

So I'm enlisting the collective help of the blogosphere in my quest. Does anyone know what the song playing through most of this YouTube video is? (The song I'm talking about starts at 0:48):



It's a pretty well-known song, I've heard it before in many other contexts, but I have no idea how to figure out the title.

Thanks in advance!

UPDATE: We're closer, but not there yet. A commenter identified the song as Lux Aeterna, by Clint Mansell. The problem is that version is strings-only rather slow and mellow -- I'm looking for the one with the cranking guitar strings. I thought that with the title I'd be able to find it no trouble -- but it's still eluding me!

UPDATE II: Found it! Contrary to popular belief, it is actually not Lux Aetnera, although that did get me on the right track. But that version, alas, doesn't have the guitars power chords. This one is a remix, done only for The Lord of the Rings: Two Towers trailer, called "Requiem for a Tower." But if you look for that, you'll only find a chopped up version by the Corner Stone Cues. Here's the real thing.

Thanks!