That's weird, I did the very same thing 2 weeks ago: I was installing a new SATA HDD in my box and I thought 'what the heck, I'll make a fresh XP install'. The original install is almost 4 yrs old and little problems were showing up now and then after a lot of game installs and removals. That also allows me to dedicate my old HDD to Linux and keep the Windows MBR untouched in case GRUB decides to mess things up.
I partition the new HDD and make an extra FAT 32 partition where I copy all my user files (and savegames) because I was convinced that I wouldn't be able to access the old user directories (marked as private).
I install XP on the new HDD and create 1 administrator user and 1 restricted user. Both names are identical as those of the old install. I try to access the old directories and I'm not surprised to see that I'm locked out. However, when I gave the same password as before to both account and when I set up both 'My Documents' as private, I was able to open the old directory for each user respectively.
It's good to know that it doesn't always work that way and that I shouldn't rely on it. Maybe in my case XP Home SP2 gave to the new users the same CPID as before.