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If you were using Windows Encrypting File System (EFS) on the drive then you would have had to export the private key prior to the reinstall. Otherwise you would need to restore a backup of the system and decrypt the files from the original user account. If you don't have either of these then the files may be unrecoverable.
Here's some information from the Microsoft site: How To Remove File Encryption in Windows XP [support.microsoft.com]