At one point, by curiosity or necessity, you need to execute a certain application you downloaded from the internet or received from a friend. Even with the presence of an antivirus on you computer, there is always the risk - the application doing something unwanted on your computer like altering a number of files or parts of registry.
The best way to protect your computer from a risk like this would be to be able to run the desired application in a virtual controlled environment that gives full access to the files on your computer but keeps changes separately, keeping the original files untouched.Sandboxie is exactly the application you would need to do this.
The application is freeware, with the only exception that after 30 days it starts displaying a nag screen to remind you of registering it.Browsing internet in a sandbox is also possible, you can do this by executing the preferred browser inside the sandbox, and once finish it is enough to close the browser and empty the sandbox.
To enumerate some of the functionalities of Sandboxie: install programs sandboxed, run windows services sandboxes, shell integration – right click on an executable –> Run in sandbox, protection at some extent against keyloggers.