How to check the CPU usage on your computer to see how well it’s performing

You can check your CPU usage on a PC or Mac using programs already installed on your computer, like the Task Manager and ...
Ars Technica

system idle process in XP@ 99% cpu usage?

If I goto the task manager it says there is a process called "system idle process" and it is always at 95-100% cpu usage. However, if I look at the graph of cpu usage it shows that it is only at like ...
Ars Technica

CPU usage=65% while idle process=99%, how?

So what's likely the cause of such behavior? A bad driver? Any way to find out which? Clearly, based on how sluggish my system is the CPU is at 65% even though the process window shows only the idle ...