That's not a feature of Windows. If you want to buy the server edition of Windows, and then buy Terminal Server on top of that, then you can do it, but you're probably looking at a couple thousand dollars when it's all said and done.
Linux and other *nix operating systems can do this, and have been doing it for many many years. On a single system it's a little difficult to do, since you'd need a second video card, mouse, keyboard, monitor, etc... For graphical apps anyway. Command line apps are easy.
On both Windows and *nix, it is possible to run different apps as different users. You might have some program running as Joe Blow, and then run another program as John Doe. Each one runs with the respective access rights of the user account, but it's not quite as comprehensive as what you are probably wanting.
Another option might be to run a VM. You could have one user account running in the VM, another user account on the host OS. Also a less than ideal solution, but might be workable.
I'd like to be able to use two Users simultaneously--that is, two Users on my PC, which has Windows 7 Home Premium, 64-bit. My hope is to not have to switch between Users (I use each User for different purposes). Can I keep both Users open AND SIMULTANEOUSLY Accessible--I mean, accessible at the same time?
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