Partition prob

hallo..
I need a help in window xp to change partition C: drive to D: because Computer's window ruining in D: drive but my all data in C: drive i did all try but can't do any thing... i also try my computer's property > Mange partition > disk management. when i writ click on D: or C: drive to change drive letter and partition it show the error like as window cannot modify the drive letter of your system volume or boot volume . pls kindly solve this solution please............