Market research firm NPD says on the basis of 12 downloads being equal to an album, iTunes sales surpassed Wal-Mart's in January and February this year.

iTunes has sold more than 4bn songs since its launch in 2003, and had over 50m customers. iTunes vice president Eddy Cue says he is "thrilled" by the findings.

The news comes at a good time for iTunes, the day after social networking site Myspace set itself up as a rival with an online ...