A. Unless someone outside of Microsoft makes a tool to create visual styles in the same format as Microsoft, you won't be able to add new skins to the OS without a third-party add-on. Microsoft has said that it won't support third-party skins for XP because the skins could cause inconsistency and confusion. If someone else makes a skin for the OS that has a 5MB title, the OS might experience problems, and people would blame Microsoft (a scenario the company, understandably, wants to avoid). However, Microsoft has provided an API that lets third parties extend this skinning capability.
Note that Stardock (http://www.stardock.com) will release Windows Blinds XP, which provides skinning; however, Stardock uses its own format for the skin and not the Microsoft format.)