It has XP.
I have managed to access the second partition (thanks for NTFSDOS and READNTFS everyone), but unfortunately that's the one with the OS files on it, not the stuff he actually needs for his work >

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Using READNTFS, it acknowledges the existance of two partitions, but it claims the first one is FAT32, and it says "Error in sector #0 or BOOT sector is invalid"
Which, to me, translates roughly to something along the lines of: "you've got a virus, haha"
I asked dad if he remembered anything wierd happening a within a couple of days of the computer not working, and he said that he got an email that didn't have his email address in the To: CC: or BCC: fields. I'm assuming that there was a virus in the email, and whatever it was has destroyed part of the hard drive.