Microsoft, which yesterday began shipping Office XP for Windows, may find it difficult to market and sell its most profitable software, according to an Upside article. The article notes that only about 20 percent of Office customers use Office 2000 (for Windows), stating that Microsoft is a victim of its own success by having perfected the software years ago to meet the needs of most. Microsoft has stated in the past that its Mac Business Unit, which centers around Mac Office, is highly profitable, signalling a possibly different trend among Mac users.