Performance tab shows highish cpu usage

Hello all, I’m running NIS2011 on a Windows 7 32 bit pc. I looked in the performance tab of NIS today and there was a period of idle with about 30% cpu usage within it but on clicking to see what had used it it showed just svchost at 1% and no other prgrammes. In the same block of cpu usage at the end of it the blue indicator had dropped but on clicking on that it said that there was ‘no activity’. I have an external hard drive connected and around the time of the idle it was spinning for a bit. This drive contains photos and music. Before this I had already run a full NIS scan, a full malwarebytes scan and a microsoft msert scan first thing this morning as I do every Saturday and all came up clean as usual. Anyone have any ideas as to why NIS shows a higher cpu usage than clicking on it reveals?