Virtual Machine

I found an awesome virtual machine thing, but I don't know what it was nor how I came across it. I remember a few features-you enabled it and it wouldn't go away until system restart, and secondly it was supposedly rated as the best at reducing memory leaks and other security threats. It advertised "try out those programs your not so sure about and see how they work in a safe environment", though not word-for-word.

Even if you can't find this exact thing, I would then appreciate what you would best recommend for a virtual machine program for Windows Vista. Thanks!