From my experience, Linux can't do anything with encrypted files on NTFS partitions, I always get access denied.
If the file's encrypted, the only people that can decrypt it is the one that originally encrypted it, and any additional people and recovery agents that were given access to it later on (I assume none in a typical home environment). This is because only those users have a key to decrypt the file encryption key associated with the file, to decrypt the file itself. Overriding permissions and/or taking ownership of the file will not allow others to get to the file, including admins. (Which is why there are recovery agents in business environments.)
The simplest solution I can think of is boot the copy of XP which can access the file, <b>in safe mode</b> so it loads with minimal drivers, log in as the user who encrypted the file, then in properties of the file in Explorer, remove the encryption. Then it should be accessible normally from the working installation from admin.
EDIT: Wait, you installed XP over itself? Into a different folder, on the same partition, or the exact same folder? If you've destroyed the user account that encrypted the file, I think you've lost the file for good.