how much you wanna bet he has a card reader, usb connected installed in the system
pretty common problem for xp to assign wrong drive letter if thats there (even without cards in it) when you reload windows..
solution.. unplug the card reader, reformat again, plug card reader back in
edit: after opening my eyes and reading original, they claim card reader in it.. so yes, that is your problem, and the computer store is right. you cannot change the drive letter of the boot/system drive