If you use the clock() function, it will return the clock ticks elapsed by your process only. My guess is that you have been using a general local-time function like "time()" to measure your elapsed times. The time() function and the clock() function are fundamentally different in the sense that clock() return the number of clock-ticks since the start of the calling process and only the clock-ticks consumed by the calling process are counted. In other words, if you use the clock function, there should be no variability introduced by having other processes running on the computer.
N.B. When measuring performance of algorithms that do file I/O, always expect variability just from the fact that file I/O is usually the main bottleneck operation and it is not very steady.
