Sandboxie

From Tzuk:

When you browse the Web, changes occur to your computer system. Some harmless, like recording the addresses of Web sites you have visited, so the browser can help you complete a Web address that you type in. Some more harmful, like the unsolicited installation of malware. When you use Sandboxie to protect your browsing session, it catches all these changes just as the browser is about to apply them into your computer system. Sandboxie does record these changes on behalf of the browser, but it records them in a special isolated folder, called the sandbox. Thus, with Sandboxie, you can browse the Web securely while still keeping all your browser's functionality for active and dynamic content, such as Javascript and ActiveX. All undesired side effects, including the removal of malware, can be easily undone.