Apple Computer has a message for Windows users considering an upgrade to XP: Come back to the Mac. In the wake of a $1 billion Windows XP marketing campaign, all eyes would appear to be turned away from Mac OS X 10.1.1, the new operating system Apple significantly upgraded in September. But the Cupertino, Calif.-based company is convinced that Windows XP"s endorsement of technologies that first appeared on Macs--802.11b wireless networking, CD burning, DVD playback, movie making, and easy retrieval of digital camera images, among others--will help Apple system and software sales. While Apple says that the differences between the two operating systems are greater than ever, at first blush consumers will see many similarities between the two products. The deciding factor may come down to the deftness of the marketing Apple employs to woo back customers who once used a Mac.