Anyhow Windows XP won't disappear so quickly as its high performance and less resource usage compare to Windows Vista. Also some PC sellers have indicated that they are going to offer Windows XP Professional by taking advantage of Windows Vista's downgrade rights. But those PCs will also include Vista installation media allowing later upgrade.
It seems that Microsoft is more focusing on Windows Vista business, while they are working on a new operating system, currently called Windows 7. Like Windows Server 2008, Windows 7 also will be built upon Windows Vista foundation, particularly on subsystems such as graphics, audio, and storage. Windows 7 is expected to be released in 2010.
Here are some images I found claiming Windows 7 will look like this...