You cannot decrypt a file by simply taking ownership and then changing the permissions of it. The same goes for putting the drive in a Linux system...it will have no effect. The file is encrypted meaning you need the corresponding key to decrypt it. That, or wait a loooong time to crack it.
Also, the local Administrator is no longer the default recovery agent on a standalone (not part of a domain) Windows XP box. If you did not explicitly create a DRA before the file was encrypted, the only way to decrypt it will to be to log in as the user that encrypted it, which will give you access to the key.
You say these files are on an external hard drive. Is it possible that the drive was attached to a different computer and encrypted there? You need to find that out. Otherwise, you will never gain access to those files.