Any standard Linux installation is configured for remote use. The traditional way of using it is with the X Window protocol, which requires X software on the client (e.g. as distributed with Cygwin). An alternative is VNC. Modern Linux distribution also offer VNC-based connections as a standard option.
Setting this up for all of your users requires some Linux/X knowledge. I have no idea what Ulteo Server does, it may save a lot of configuration and troubleshooting effort. Similar offerings may exist from other vendors. But the basic capability of working remotely, with multiple users at a time, each with their own desktops on the same machine, is standard in Linux.