The BIOS backup is there specifically for situations like that
Anyways, if you had the data encrypted using the WinXP NTFS encryption system, then yes your data is gone forever UNLESS you created a password backup disk (which nobody does)
However, if it was just a profile that no one had access to then it is *possible* but not easy, I have only done it a few times and its a pain in the ass, especially if you don't know what you are doing.
If he installed XP Home on the HDD without formatting, he would have been well aware that XP Pro was on there (it tells you during install)
Have fun with the profile stuff, unfortunantly in most cases the data is better off left to die, and the only (supposedly) easy way to get it is if you are a Linux guru but I still don't understand how they do it....