Thin clients are a great way to repurpose older PCs or get cheaper systems and still get a powerful network of workstations for schools or institutions with many different cubicles/offices for a fraction of the cost, both in cash, time and staff invested in its deploying and maintenance. However, just like any other computer, there are details that have to be worked out here and there. Tonight I'll give a tip about a particular implementation: LTSP plus Windows virtual machines with concurrent sessions over RDP (tested with Ubuntu 12.04 as a base OS and Windows 7 Ultimate as guest OS under Virtualbox).
One thing you'll want is server uptime as close to 100% as possible. After all, without it, no workstation will be usable. However, one issue I often ran into was that the Windows virtual machines would close on their own. After some search online, the only clue I got is that the Virtualbox logs report that THE GUEST OS REQUESTS THE SHUTDOWN. This does not tell us much, because, after all, what could be causing Windows to issue a shutdown command? Well, the answer is: Virtualbox interprets any suspend/hibernate and possibly screen power saving as a shutdown call from Windows, hence terminating the virtual machine.
The solution? You need to disable any and all suspend/hibernation/screen suspend timers under Power & Energy saving. I happen to lack the english MUI on my Windows 7 installation, but what you'll want to do is enter a power scheme's configuration page, and click the link that will bring up an "advanced settings" window. In there, just reduce the time in minutes for shutdown/hibernate/screen suspend until it displays "Never" instead of a digit.
This should save you a lot of headaches with your setup. It worked for me, and the school that I sold my first LTSP network to is already working on the budget for another classroom with thin clients.
Hope this post helped you. Have fun testing, and never give up.