CPU Usage doesn't add up in Task Manager

I am a very confused non-technical person and I am hoping someone here can help me.

My computer has been really slowing down of late, CPU usage regularly hitting 75-100%. First thing I did was the basics: defragged, updated and run Norton AV, then scanned my computer the free scan on the McAfee site, updated and run Spybot S&D, updated and run Ad-Aware -- all have come back saying the machine is clean.

When I went into Task Manager, it was showing no suspect processes running and no processes running at superhigh level for long periods of time. But it did show something *really weird* which I don't understand: the CPU usage numbers just didn't appear to add up at all! The CPU usage of the ndividual processes did not add up to the total CPU usage showing at the bottom of the Task Manager by a considerable amount -- basically, the sum was showing as greater than its parts. It was as if the CPU was running at a higher (slower) level than was justified by the processes running. Or as if there was some process using up some of the CPU but not being listed by the Task Manager.

Because the Task Manager is constantly changing, it was a little hard to be sure I was reading it correctly, so I took screen-shots to check again in slow-time and make sure I was seeing what I think I was seeing. I took 5 screen-shots with the results as follows:
Screenshot1: Total CPU Usage = 70%, System Idle Processes = 53%, Other Processes = 29+8+3+3+2+2=47% Mysterious Missing CPU Usage = 70-47=23%
Screenshot2: Total CPU Usage = 67%, System Idle Processes = 67%, Other Processes = 19+5+5+2+2=33% Mysterious Missing CPU Usage = 67-33=34%
Screenshot3: Total CPU Usage = 83%, System Idle Processes = 33%, Other Processes = 46+6+6+4+4+2=68% Mysterious Missing CPU Usage = 83-68=15%
Screenshot4: Total CPU Usage = 84%, System Idle Processes = 39%, Other Processes = 48+5+2+2+2+2=61% Mysterious Missing CPU Usage = 84-61=23%
Screenshot5: Total CPU Usage = 67%, System Idle Processes = 55%, Other Processes = 20+7+5+5+3+2+2+2=46% Mysterious Missing CPU Usage = 67-46=21%

Can anyone explain to me why the numbers seem so wrong? Is it normal, or is it a problem? Is this the cause of my computer running slower these days? As I said, I am a totally non-technical person just feeling my way in the dark, so if these are stupid questions or something glaringly obvious, I apologise. I've seen another thread that touched on this, but didn't seem to answer my questions. I do appreciate any help anyone can give.


PS -- computer details: Pentium3, WindowsXP Home Edition 2002, 256mb RAM

PPS -- In addition, in the last screenshot, the System Idle Processes percentage seems to bear no relation to the CPU usage of the other processes. To my understanding, the System Idle Process percentage should be the difference between 100% and the amount of the CPU being used by active processes -- ie, if Excel is running alone at 25% CPU usage, System Idle Processes should show 75%. Am I right in my understanding?

 

Actually I think System Idle processes are programs waiting in the background to run. For instance you might have MSN Messenger 6.2 set to automatically sign on when you connect to your ISP. The autodetect feature would probably be one of the idle processes. Also I believe multiple running tasks increase CPU usage exponintialy. For example running IE by its self at 10% and say Messenger 6.2 at 5% may = 22% total.

I think this is due to the cpu having to jump between multiple processes, back and forth a little at a time to keep them running and doing what they are suppose to. Your heavy CPU usage may be due to too many active processes running at once. Try turning off some of the programs you don't need running. Also see how many Icons you have in the system tray on the bottom right. Too many of these will cause slow down and heavy cpu usage.

I believe Hyperthreading helped remedy this in the newer processors. Also you may want to look into adding more system memory. More memory is better, it may not cmpletely solve the problem, but it will help.

Try some of this and let us know if any of it helps

 

System Idle processes never uses CPU time aggressivley. System Idle processes do not get taken into account in the overall usage in the same way as initialised processes. This is because the Overall usage is calculated with compensation for the number of useful System Idle instructions (a lot of them are HLT commands, which tells the processor to do nothing for a tick, which produce no calculated result).

The total % usage should add up to 100% including System Idle, and if you look, it does (47% & 53%, 67% & 33%).

What I'm trying to say here is: yes, that's normal

 

Thank you for correcting me goldfish. I wasn't sure how the percentages were tallied.

 

Thanks both of you for replying so quickly.

Goldfish, I agree that those numbers add up, but at that same moment, the total CPU usage figure at the bottom doesn't add up (for instance, in the first screenshot: 47+53 = 100, but CPU Usage is showing as 70%). Do you know why this is? By rights, I should be operating at 47% usage (and the quicker speed that gives), but my CPU is slowing down because it seems to think it's actually running at 70%, when the processes just don't add up to this. Where is it getting the 70% from?
...Confused.

tigerray, I would like to shut off unnecessary processes... I would like to add more memory... but the first scares the life out of me and the second is beyond my understanding. So, I look at the Task Manager, try to make sense of things and get all het up when it doesn't seem logical to me.
Learning ...slowly!

 

It should be noted that task manager does not see all processes running.

 

I would suggest going to System Utilities in downloads and getting Process Viewer or something similar and at least looking for yourself what is going on in there.

If it doesn't overwhelm youir capacities, try Everest home edition. Very good prog.

 

There is a way to find out what processes are running. When you first log in to your computer, before you do anything check your start-up folder, and access your startup programs in system configuration utility(I'm not sure what the win xp equivalent is) and check to see what you have automatically starting.

A lot of programs will set themselves to automatically start at log on when you install them it may be that some of these are unnecessary until you actually need them.

Check this and let us know what you come up with.

 

Anything that shows up in the startup folder or even the run sections of the registry will show under task manager once they are executed.

However, there are many things that don't show, as well.