MCP in windows xp get MCDST certification from Microsoft and it is in good demand. employers are looking for MCDST technician.many organizations are still working on windows xp.

MCP vista is going to release, rite now new generation certification is dere as MCTS windows vista configuration. MCTS: Windows Vista – Configuration certification is designed to validate the knowledge and skills necessary to understand the functions and features of Windows Vista and to troubleshoot network-connectivity and applications issues.

both the certifications are good and dere is no difference except the windows, both are good in demand n industry recognized certifications.
As an MCP, it specifies under which OS you are certified. Of course, Vista being the newest OS, it specifies that you have been certified with it. Although being certified an MCP under XP is still valued.