Cloud radio access network (C-RAN) is perceived as an energy-efficient solution for the next mobile network. The cloud-based baseband unit (BBU) pool is capable of dynamically provisioning computational resources for mobile users to improve hardware utilization such that unused processors can be switched off for power saving in the BBU pool. Besides, cooperative radio among remote radio heads (RRHs) can optimize transmit beamforming to reduce power consumption at RRHs. Thus, to achieve more judicious system power saving, this is need to consider power consumption in the BBU pool and that at RRHs together. In this paper, we aim to minimize system power consumption by jointly exploiting transmit beamforming and processor sleeping. Specifically, we formulate a mixed integer non-linear system power minimization problem, which is hard to solve. For tractability purpose, we transform this problem to an equivalent clustering problem embedded with a series of transmit beamforming problems and processor sleeping problems. On this basis, we first focus on solving the embedded problems with the given clustering and then propose a low-complexity clustering algorithm to search out the optimal clustering with minimum system power consumption. Finally, simulation results show that our proposed algorithms can save system power significantly.