This work reported in this paper exploits a network of inexpensive wireless sensors to improve the energy efficiency of desktop computer usage in a major business or campus environment. Computer users frequently leave their desktop machines permanently switched on or in sleep mode when finished using them, in order to quickly resume work when returning. This results in unnecessary power consumption for a relatively small return in terms of user convenience. Eliminating this unnecessary power consumption without sacrificing user convenience is what our research achieves. The novelty of this work is that it achieves this goal using ambient sensors to establish user context.