If the 40gig disk previous had data on it, but you installed XP (and did a format in the process), some of the old data will still be recoverable. Most people probably wouldn't bother trying a recover, especially since they don't get a "blank" disk (ie., if you had just formatted it but not installed XP, or if you had deleted the partitions or whatever).
I guess what to do depends on what kind of data you had on it. In a similar scenario, I wouldn't bother - (almost
) all my sensitive data is encrypted, so at most a badguy would be able to tell my browsing history, see some (non-xrated) picture of me and my friends, stare puzzled at passwords.exe (yes, I drink my own kool-aid
), et cetera.
If you had credit-card info or similar lying around in plain.txt files, you might want to call them and get the disk back, though... just in case.