Decrypting encrypted files from old windows

Install new windows from the same partition drive but the other partition is lock or encrypted with the old user, for somehow i back-up the user profile but how can i install the certificate from the new os?

if i were to copy the old \Documents and Settings\User Folder to the new installed windows xp how can i be able to obtain the certificate and install it to the new user or make it as recovery agent to remove the encryption.

Please help me to troubleshoot or make a way just to recover very important file..

Thanks..