A huge amount of heavy metal is influenced by classical music, particularly black metal (Emperor), Power Metal (Therion), NWOBHM (Iron Maiden), and bands like Metallica and Opeth (I think there's a madrigal on one of their albums). On the album "How The West Was Won" by Led Zeppelin, during Jimmy Page's solo on the song 'Heartbreaker', he plays a bit from one of Bach's Lute Suites.