I'm trying to figure out what's going on with my PC. It seems the CPU usage is WAY higher than it should be. I've been investigating, but can't figure it out.

When I look at the CPU usage chart (or the little task manager CPU usage indicator in the system tray), the usage is way high. Close to 100% almost all of the time. Looking at the CPU usage chart on the performance tab of the Task Manager, I see that the CPU is often maxed out, and rarely (if ever) drops below 70%. This makes sense, as the general response of the computer is sluggish and way slower than it used to be. When I look at the 'Processes' tab of the Task Manager, however, it tells a different story. Here's a common glimpse into the CPU usage rates that I see (for example, as I type this message):
System Idle Process: 80-99%
taskmgr.exe 0-17%
explorer: 0-20%
CCAPP.EXE: 0%
zlclient.exe: 0%
etc.
And lots of others that never get above 0%

To me, given these numbers, I would think that the usage chart would read differently, and my computer would actually respond promptly! But this is not the case.

I've run a virus scan, I always have my firewall up (Zone Alarm), I ran a few different Spy/Adware scanners -- they all turned up nothing.

The only software I've installed recently was Norton Antivirus. I was using AVG before. The thing is, I don't know if it's Norton or not. I don't see any of its processes show up high in the list of CPU usages, and the software worked very well for the first couple of months that I used it. So, why now?

Anyway, if anyone has some suggestions on what might be going on, how I can further diagnose the probem, or what actions I can take to fix the problem, please share. Thank you!

When a computer is Idle it will have a high System Idle process under the CPU often in the 90's. This is completely normal for a computer as it waits for something to do. If you leave the Task Manager open a program you sill notice that the System Idle process decreases its CPU usage. It will be high when you are doing something little, browsing a simple webiste. When your doing something with more CPU, like downloading music, running JAVA it will take some CPU off "idle" and use it for another program" there must always be 100% CPU running.