If this is for Linux/unix, you don't need any special software. Multiple users can login to a single Unix box simultaneously.
If this is Windows, Microsoft prevents you from doing that unless you buy a separate Windows license for each desktop. The official business way is to pay for terminal services licenses. That'll make your Windows box act like the Unix boxes do - with multiple people able to login and use the same physical PC simultaneously.
But if you just want one person to use the PC with two desktops, I'd try Virtualbox, VMWare, or Virtual PC. Virtualbox is free, and has the most functionality of a free version. I'd say VMWare Workstation has the most features of all the options, but it's a couple hundred dollars. VMWare Player is free, but has been stripped of most of its features.. And if you have Win 7 Pro or Ultimate, you already have a copy of Virtual PC. But it's limited to Windows as guest OS.