Sandboxie runs your programs in an isolated space which prevents them from making permanent changes to other programs and data in your computer.


Secure Web Browsing: Running your Web browser under the protection of Sandboxie means that all malicious software downloaded by the browser is trapped in the sandbox and can be discarded trivially.
Enhanced Privacy: Browsing history, cookies, and cached temporary files collected while Web browsing stay in the sandbox and don't leak into Windows.
Secure E-mail: Viruses and other malicious software that might be hiding in your email can't break out of the sandbox and can't infect your real system.
Windows Stays Lean: Prevent wear-and-tear in Windows by installing software into an isolated sandbox.

Ref and Download: http://www.sandboxie.com/

It is handy when you dont trust a certain software you have obtained from someone or web;
Hope it was useful