To begin with, the Atom N450 is a pretty weak CPU. It is a single core CPU with Hyper Threading which means it as one "real" core and one "virtual" core.
As suggested by rgd1101, you should use Task Manager to see what processes are running when the laptop is simply idling. If there is a hard drive activity LED that is always flashing, then it is possible that there is a lot of data swapping between RAM and the PageFile, upgrading the RAM can alleviate this, but you should first see what is causing the CPU to be idling between 50% - 100%.
It is a pretty old laptop since it was released sometime back in 2010 I think. It is possible the Windows Registry may have become bloated over time and may be the reason why the CPU is running at 50% - 100%. CCleaner can be used to clean up invalid entries in the Registry, but the best way to ensure the Registry is not bloated is to reinstall Windows.