Sandboxie is a tool which allows you to run programs in a secure environment on your windows PC.
Once installed it adds a new option in the right click menu which allows you to 'Run Sandboxed' which means you run any program without it writing to the hard drive and preventing you from getting infected with all kinds of nastiness.
If you so choosed you could download any of the worst virii out there and aslong as you ran it within sandboxie you should be completely safe.