Well if you have other programs running then the result will be: The drives maximum perfomance minus the perfomance that any programs running are currently using. So the test result will vary every time since any single program could have a varying cpu usage depending on what it is doing during the test. e.g. your antivirus program could suddenly start running a scan halfway through the test - this would lower the results of the benchmark so the result would only have meaning if your anti virus software was scaning your pc 50% of the time.
However if you want to know what kind of perfomance your drives are actually giving then you should run the test wth as little as possible running.