In order to use Windows from your ltsp thin clients, you need to edit your lts.conf file so it contains this line:

SCREEN_02 = rdesktop yourservernamehere

Then when the thin client boots up, you’ll be faced with a Linux login window. But hit ctrl-alt-f2 and you’ll be faced with a Windows Terminal Server login. Ctrl-alt-f1 gets you back to Linux. You can use both systems simultaneously.