It's the Lacrimosa section from Mozart's Requiem Mass.
Which Commercial?
Cannon by Pachelbel.
The background music has no known name. It was commissioned for the tamiflu ad and produced by facethemusic.com
a version of ride of the Valkyries
Flight of the Bumblebees
Bach
the singer in the background is Enya
Pachelbel's Canon.
ladies and gentlemen - saliva
poisoned well
Most music for commercials is specially composed for each situation. It may sound similar to something familiar, but may not be exactly the same due to copyrights, performer royalties and the like.
Clair de Lune - Debussy