How to decrypt data in ntfs

[UncleGemboel]

I have big problem here, my friend without notice has decrypt some of my important files and when i reinstallled my xp i can't access that files anymore, i know i must have specific keys in my previous installed xp.
So is there any chance i can get my data again? How to decrypt that files?
Thanks for the help.

Note : i format my hd when reinstalled my xp, files on the other drive, service pack1.