Can't access the encrypted files after the format

Hello
This problem with my friend PC, he was using winXP, there was plenty of encrypted files in his D drive, now he formatted his PC to win7 ultimate, all the encrypted files are unaccessible, he don't don't know whether he backup the certificate or not, he know nothing about this, is there a way to access these files without the certificate? and if he backup the certificate where is the default location that it goes? or the extension of the certificate? maybe I search and find it.

All files of any importance need at least one backup copy, 2 or backup copies if the files are of particular importance. Encrypted files are no exception. You just need to take proper precautions with the backup media. But it is too late for that now.

Thank you all, just like you said backup of encrypted files is something you have to do, the PC is not mine its for my friend and it seems that he did not backup the certificate after he searched on his computer.
Well, I told him that the files now are useless, but recommended him to not remove them.