Most likely a bug. My system is no beast but it is also no slouch. Just doing a full system scan while doing nothing else. This is with Windows 8.1 with latest updates btw. I seen as much as 71% cpu usage! It mostly stayed around 50% or slightly more. I was also seeing ram usage shoot up. At one time using 3.1 gb. Anyone else seeing this? Try a full scan if you have not and see how it goes.
System
i7 2600
8gb 1333 ram
evga 2gb superclocked gtx 660
160gb intel 320 ssd
500gb sata III hdd