Microsoft is expected to enter the online song store market this week, which should put the software giant head-to-head with Apple Computer in the music business at last. The launch of <A HREF="http://zdnet.com.com/2100-1104_2-5327631.html?tag=zdnn.alert">Microsoft's iTunes rival</A> will be timed along with the beta release of Microsoft's new Windows Media Player 10, expected on Thursday.