you can try using linux redhat. During startup, you can choose the OS you have installed and wanted to use.
check out their website before choosing.
You may try running a powerfull linux server that is running virtual machines with winXP in them.
Linux allows the kind of logins you want (that's the reason for the whole mount/unmount thing)
User logs into the linux server, server asigns them a virtual machine to use.
I guess you want one computer so you just need to patch that one and install software on that one etc. If you prefer Windows (wich I read in your post) I would suggest a Windows terminalserver. You said you wanted winXP or win2003, I thought there is a terminalserver edition from win2003. On that terminalserver you can create an account for every terminal/user so every user thinks he has his own pc and you can access them with the remote desktop program you mentioned. But be sure to use different users for your different computers, else the fisrt user will logoff when the second user logs on.
This login from terminal method used to be very popular (back when computers filled rooms).
You may get success by researching into these very old systems.
I know about thin clients. But I want to know, whether it is possible without thin clients or not. I want to do the same thing, what a thin client do. I want to use resourses of Main computer by login in it through different terminals.