Sandboxie

Sandboxie is a sandbox-type isolation software for the Microsoft Windows operating system. It creates a "container" in which programs can be safely launched without modifying the host's OS.

When you run a program on your computer, data flows from the hard disk to the program via read operations. The data is then processed and displayed, and finally flows back from the progam to the hard disk via write operations.

Sandboxie changes the rules such that write operations do not make it back to your hard disk.

• Intercept changes to both your files and registry settings, making it virtually impossible for any software to reach outside the sandbox.
• Traps cached browser items into the sandbox as a by-product of normal operation, so when you throw away the sandbox, all the history records and other side-effects of your browsing disappear as well.

http://www.sandboxie.com/