On my Windows 7 PC, the free disk space has gone down by 1GB even though I haven't downloaded or installed any new files and I haven't downloaded any updates or other things? What could have made the disk space go down for apparently no reason? Is this the result of some sort of a spy program that is undetectable?

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    You could use Windows Search to look for any large files (say > 100MB) created or updated recently (e.g. in the last week). – RedGrittyBrick Mar 21 '11 at 11:03
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    Do you have IIS installed? Or apache? Or any program that generates log files? – Viper_Sb Mar 21 '11 at 13:54
  • Sometimes Windows 7 does defragmentation of drives automatically, leading to apparently random increases/decreases in disk usage. – apoorv020 Mar 26 '11 at 14:17

Use WinDirStat, a freeware disk usage statistics viewer and cleanup tool for Microsoft Windows to see what's filling up your harddisk.

Windirstat screenshot

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    +1 for WinDirStat - it also comes in a portable flavour. portableapps.com/apps/utilities/windirstat_portable – ta.speot.is Mar 21 '11 at 11:43
  • +1. I use WinDirStat all the time on servers here to find out where all the drive space is being eaten up. – Hyppy Mar 21 '11 at 12:12
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    And if you have "Unknown space" in windirstat check if you have backup software running. notable offender is thinkpad backup... – bubu Mar 21 '11 at 14:37
  • lol thanks for this, just cleaned up over 20GB of trash I forgot about over the time :) – sdadffdfd Mar 21 '11 at 17:40

It could be any number of reasons. My Windows 7 directory has steadilly grown over the months from 12 Gig to 22 Gig (mostly system updates).

Possible causes:

  1. Your virtual memory page file has grown.
  2. System restore snapshots have been created.
  3. Windows update has installed updates automatically.
  4. Windows is just doing what windows does best.

Which of these it is, who can say?

I would recommend jDiskReport from jGoodies which will show you what is using the space on your hard drive with some nice pretty pie charts.

  • Boo not WinDirStat! – ta.speot.is Mar 21 '11 at 11:44
  • No, not WinDirStat. jDiskReport is much nicer for the novice user. WinDirStat, while better, just melts your eyes before you can understand what it is telling you if you don't know what you're doing. – Majenko Mar 21 '11 at 12:03
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    The j stands for java. – ta.speot.is Mar 21 '11 at 12:09

As Matt said, there are too many reasons that could steal your disk space, specially on windows. Yes, it could be from a spyware or virus, but picture that issue as a small computer fever and as such it's not a good diagnosis source.

For citing another reason not cited yet, back in the days, disk fragmentation were a big issue. I'm not sure if it holds true today, but Microsoft still advices on doing it in windows 7. So go ahead and defrag once in a while, paying attention ideally you should have 15% free space before doing it.

I love WinDirStat, from splattne's answer, and it is indeed a perfect tool for analyzing current disk usage, but not for finding a sudden free space lose. It'd be much better to follow RedGritty's advice and do a windows search for big files in the last days - or even mixing both. Don't forget that log files do grow up as well and are a very common responsible disk space eaters, like Viper said.

Once you find the culprit, there's a good chance you could make good use of msconfig to "removing" the source of the issue. I used it for years on my late windows machine and no anti-virus or any kind of malware protection at all.

So my advice to find it, is to use every tool at hand. There's no single tool able to help you with such a broad issue. Another tool highly overlooked in every OS is the log analyser. Console on mac, /log on linux and Event Viewer on windows. It can give you some clues on what might be happening with your system, if you're lucky. On windows it is overlooked for a good reason: most of times it doesn't log enough info.


Another source of the increase is having Windows Previous Versions turned on. This feature saves copies of system and user application files (e.g. Word) whenever you change them. You can then recover back to a previous version if you need it. Go to Computer|Proerties|Previous Versions to see what you have. This facility can be turned off.

  • Previous Versions uses restore points (shadow copies) to store older copies of files. As a result, a file deletion often does not reclaim space, and changes to files that were unchanged since the last restore point will result in a copy of the original reducing free disk space. – bwDraco Apr 7 '11 at 13:30

Yeah the problem was system restore images being created and the space did always seem to go down after I put some new files on the computer or installed something. I checked the options and the limit was 10 GB and I've decided to turn them off as I don't really think I need it. Anyway thanks to everyone for their time in answering, my faith in humanity has been restored!

BTW Procedure for reclaiming this space:

  1. Go to Start > Computer > Properties

  2. Click on System Protection

  3. On the System Protection screen choose Configure

  4. On the Disk Space Usage press on Delete to erase the old restore point and then reduce the Max Usage to 10 or 5GB. Press Apply and OK.

Make sure you explicitly create a new restore point if you want to have one.


A common stealer of disk space is the windows side-by-side (winsxs) assembly (see these links:Side-by-side Assemblies , misreporting)

This is a method used by windows to allow multiple library (DLL) versions to coexist on a computer system without the incompatible DLL problem which plagued older versions of windows. My limited understanding of it is that many of the assemblies are virtual, and in some cases Explorer will double count disk space usage, leading to misreports of used space.

Windows 7 seems better than vista in this regard, but I personally saw my winsxs folder on Vista rise to about 15 GB. I actually ran out of disc space on the OS partition because of this.


C drive is losing space without any reason on win 7

What to do

  1. Go to start>computer> proprieties

2.Click on system protection

3.On the system protection go to Configure

4.On the Disk Space Usage press on Delete to erase the old restore point and then reduce the Max Usage to 10 or 5GB.Apply and ok.

  1. Last thing to do is to create a new restore point if it's done you will see that you disk space will increase.
  • This should have been a comment under Dave's answer. I have added you text to it. – Jan Doggen Dec 15 '14 at 9:38

I have the same issue as my C drive filling continuously without any reason. Whenever I refresh C drive properties after a 5 Minutes of interval disk size would full by 100 MB. I delete 10 GB data and in couple of hour again C drive full. I turn off System restore feature but it doesn't helped.
Then I diagnose C drive space by WinDirStat then I realized that SQL Server Reporting services creating the Dump file which filled my drive space 100+ GB and continuously filling up.

WinDirStat statistics of C drive

I stopped all the Reporting services and deleted dump.

  • Can you explain to OP how to detect the cause? – yass Mar 28 '17 at 16:25
  • @yass Have a look at WinDirStat statistics of C drive attached image, you will see that Reporting services has consumed 101 GB space. – D_K Mar 30 '17 at 5:52

You could also clear cache. There might very well be up to 1 GB space used. Though that wouldn't be a sudden decrease.

  • It isn't clear to me whether you are suggesting that the cache is the issue ("might very well be up to 1 GB") or not ("that wouldn't be a sudden decrease"). – Adam Mar 28 '17 at 22:06

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