SLOW XP,TOO MUCH RAM

I installed XP last year and later upgraded my PC to 256Mbytes RAM. My system is dead SLOW! Starting up is max 20 mins. Recently the my applications are non responsive and respond after a while. I have about 3GBytes free space and running on a network. I have thought of downgrading the systems to WIN2K Pro, how to I do it. How do I downgrade