Monitoring the performance of a computer based on the value of a net useful activity metric

A computer implemented method of monitoring the performance of a computer comprises determining S20 the total value of an activity metric of the monitored computer. The contribution to the total value of one or more predetermined activities is determined S22-S28. The contribution is subtracted S30 from the total value to provide a net value representing a measure of the performance of the computer. Similarly, the total values of a plurality of activity metrics may be determined, to provide respective net values representing a measure of the performance of the computer. A data set held in a data store may be used to identify the one or more predetermined activities. An activity metric may be CPU activity or a value of input/output activity and the predetermined activities may be processes. An activity metric may be the number of active network connections or the number of logons. The net value may be used to control the power consumption of the computer.