With online music stores popping up all over, Microsoft has decided to build a mall for them. Windows Media Player 10, slated for public debut this fall, will offer a collection of digital-media retailers under one virtual roof. Rather than having to download several applications, then jump from one to the next in search of content, users will be able to drop in on various vendors while remaining in Media Player. Any music or video bought in the mall will be collected in one place and will be compatible with the player and supporting hardware. Right now, it's easy to misplace songs from assorted stores or misunderstand how tracks from different stores can be played.