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On Windows 7, the System process is taking up 100% CPU (well 50% on a dual-core) for a rather long time. Its CPU usage doesn’t seem to drop at all.

How can I diagnose this problem? What could be the cause? I don’t see any other problems with the system and am using an up-to-date copy of AVG, so I don’t yet believe it is malware.

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I am getting this, and it's a bit disappointing that none of the answers are that useful. – mackenir Feb 8 '11 at 12:25
@mackenir - upvote if you think this is a useful question. You can also favorite it. – ripper234 Feb 8 '11 at 13:32
up vote 3 down vote accepted


Drivers (whether hardware or software) are loaded into the System process, so usually, when it is using a lot of CPU cycles, a driver is misbehaving (possibly in a deadlock or just stuck in a bug).


To identify the culprit, get a copy of Process Explorer and double-click the System process. Switch to the Threads tab and sort on the CPU column and look at the module(s) using up the most CPU time.


Next, you can view the properties of the file and/or Google the filename to determine which driver is malfunctioning.

At this point, fixing the issue depends entirely on the driver.


In this case, CPU is not being pegged, but this snapshot (figure 1) shows the top users at this moment are iaStor.sys and MpFilter.sys. If their CPU usage was high, we would look at the files to see that the former (figure 2) is a driver for an Intel drive-controller and the latter (figure 3) is a component of Microsoft’s anti-malware program.

If the Intel driver were problematic, one could check to see if there is an update, if the drive is faulty, if there are others complaining about the same problem in the Intel boards, etc.

If the MSSE file were problematic, one could check for an update, if others have the problem, uninstall it, etc.

Figure 1: Screenshot of System process threads Screenshot of System process threads

Figure 2: Screenshot of Properties dialog for iaStor.sys iaStor.sys properties dialog

Figure 3: Screenshot of Properties dialog for MpFilter.sys MpFilter.sys properties dialog

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Modern Antiviruses are a little rubbish, don't assume you're fine because you've got one (though they can help)

The System process taking up an entire core shouts "Driver issues!" quite loudly to me, anything been updated or changed recently? Any devices showing a bit iffy in Device Manager?

Grab a copy of Process Explorer and take a look at what it's doing in a little closer detail.

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Nothing was changed in the system, I tried peeking with process explorer but didn't find anything. A reboot solved the issue for now. – ripper234 Oct 31 '09 at 15:49

You first need to find out which system process it is. Go to Task Manager, Processes tab, and click twice on the column "CPU Time" to sort descending (if it's not there, add it from the menu View / Select columns...).

If it's by any chance the "System Idle Process", then this is quite normal.

If not, right-click on the system process and choose "Go to service(s)". This will open the Services tab and highlight one or more services. Please include their names (or a screenshot) in your post for us to see.

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There is this interesting article, if you'd wanted to know what exactly is causing it.

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Huh? What about the System Idle Process? This is included in the Process tab so that the relative percentages will add up to 100%. – davidcl Oct 31 '09 at 13:33
Whoa, thank you sir, you are right. I was blind but now I see... I don't know why I thought it was the way I explained it. – sYnfo Oct 31 '09 at 13:48

This could be a process doing heavy I/O. Since that is performed by the System "process" you would see the CPU usage spike there.

I've once had that with the IDE controller reverting itself to PIO mode. Though that was on an ancient system, dunno whether that even happens anymore with modern OSes :-)

Also don't forget that AV software can even cause this themselves. It's not uncommon for them being a performance problem :-)

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Try disabling System Restore Point : Control Panel\System Then check if the system is consuming 100% CPU cycles. If Yes, defragment the Hard Drive (force defragmentation even if Win7 says it is OK) If yes, uninstall AV/FW using proper uninstaller program from AVG ( Re-install new AV/FW and check if CPU is running at 100%.

Try following spyware removal using Malwarebyte's Script (

Download/install GMER and provide GMER Log (

Provide HijackThis and Speccy logs.

Many a times, when Windows is trying to update, the corrupted AV/FW can interfere with Windows Update. Badly updated Windows can also wrack havoc on CPU usage time. Once everything works, re-instate System restore Point.

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