In corporate networks, idle desktop machines hardly ever sleep, mainly for the reason that turning off the hosts will affect the internet connection. There are number of systems have been proposed, few of them have mutually considered energy consumption and performance. In this article, we consider the case of file downloading, which is one of the most common internet applications. A system called College-Net is proposed for saving energy while improving performance by exploiting task migration and redundancy reduction. With this College-Net system, file downloading will not be interrupted even when a PC switches to sleep. In particular, it allows a PC to sleep while continuing to run large file downloads. Moreover, by caching the downloaded files in the server, College-Net suppresses the reduplicate downloading effort and reduces the network traffic. The strategy of redundancy reduction used in College-Net improves the average speed while avoids the energy waste caused by repeated transmissions of downloaded files. The proposed College-Net does not require any hardware additions to end hosts, and can be realized purely by additional software.