In this paper, we present a new approach for safe execution of untrusted programs by isolating their effects from the rest of the system. Isolation is achieved by intercepting file operations made by untrusted processes, and redirecting any change operations to a "modification cachel" that is invisible to other processes in the system. File read operations performed by the untrusted process are also correspondingly modified, so that the process has a consistent view of system state that incorporates the contents of the file system as well as the modification cache. On termination of the untrusted process, its user is presented with a concise summary of the files modified by the process. Additionally, the user can inspect these files using various software utilities (e.g., helper applications to view multimedia files) to determine if the modifications are acceptable. The user then has the option to commit these modifications, or simply discard them. Essentially, our approach provides "play" and "rewind" buttons for running untrusted software. Key benefits of our approach are that it requires no changes to the untrusted programs (to be isolated) or the underlying operating system; it cannot be subverted by malicious programs; and it achieves these benefits with acceptable runtime overheads. We describe a prototype implementation of this system for Linux called Alcatraz and discuss its performance and effectiveness.