A Virtual Computer is where you install Virtual Machine software onto your computer and then it's basically a computer within a computer.
The virtual machine or virtual computer allows you to run a separate version of your operating system on your computer which is completely isolated from your real operating system.
So if you use the virtual machine for testing something and if something goes wrong on that part of the computer it won't affect your computers actual operating system.
You can easily roll back the virtual machine even if something such as SYSKEY happens to it.