A remote desktop offering. Most windows versions (all? I'm not sure about that, but the professional and server versions sure do) come with builtin RDC, called mstsc. There are also 3rd party alternative solutions like Citrix.
If additionally you want each user to be able to work on different OSes on the same CPU, use a virtualization application like Virtualbox or VMWare server.
Edit: No strike that - I misunderstood your problem. You'll still need separate boxes even when using RDC. However, the system you describe (with just one box) was used by NIIT among others if I remember right, and a google search shows they used an offering from
http://www.ncomputing.com/. It requires specialized hardware to allow multiple keyboards, mice and display outputs - that's what they provide.