The `System Idle Process` is exactly what it says - the amount of CPU not in use. I think for most people, unless there is something running this will show 99%, the fact your is showing 48% shows something is running and the CPU usage proves it.Make sure the `Show processes from all users` is checked and scroll down the list to find what is using the CPU.
There is a difference between CPU and memory usage; you started by referring to CPU so I presume you mean this still.
The processes listed are those running on your PC. If you click the CPU box at the head of the column it will list them by those with the highest CPU usage downward (click again and it reverses this). You can do the same for memory or any of the other columns to give alphabetical order/user.
As stated, it is not the Memory you should be looking at, but the CPU Percentages - normally the Percentages are mostly 0% but something like Adobe updater can use very much more, the only Percentage which normally shows at about 94%-99% is System Idle Process which is good.
Any other process using 50% or more should be investigated.
Whatever is using up your CPU Google it and it will tell you if it is anything to worry about.If still not sure post it on here for us to have a look at.It may be a scan running or one of your programs up-dating.