Ok, the user's account, for EFS, has a x509 certificate and key associated with it. if the xp user account was a local one, then your only hope is that you can recover that keystore from an unformat of the laptop - assuming that win7 has been applied, I would not hold out a great deal of hope there though.
there is no other route - EFS is not easily crackable (despite a certain American newspaper claiming they broke it in 3 days :) and almost all known attacks are on a password protecting the EFS key, not the encryption itself. I would therefore *immediately* discontinue use of the old laptop, remove the hard drive, mount it on another pc via a usb adaptor, and scan it with AEFSDR - which isn't cheap, but presumably cheaper (if it works) than losing the files. AEFSDR Pro can scan hard drives for the keys it needs, which if you are unreasonably lucky will be enough.