Unexpected free disk space decrease?

Solution 1:

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

Solution 2:

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.

Solution 3:

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.