I haven't personally ever partitioned a drive, but I believe the best thing to do would be to create 3 partitions....
1 partition for the Windows (whichever distro) Operating System
1 Partition for User A
1 Partition for User Z
No matter who starts the computer, Windows will be loaded from the OS partition, and you can have 2 users set up in Windows, and then just give each user access to thier own other partition... User A would have access to his partition, but not user Z's partition... and vice versa for User Z.
But, what is SOUNDS like you want to do is a dual boot, so that you pick which OS is booted, but I would imagine it might be confusing if you use the same OS as the dual boot... I also have no experience with dual boot systems....