New Windows 7 feature in KB 2852386 reclaims wasted space on your C: drive
Think of the WinSxS folder as a Windows elephants' graveyard. When Microsoft patches Windows, it sticks the obsolete versions of earlier files and their settings in the WinSxS folder -- typically C:\Windows\WinSxS. That way, if you want to roll back a patch, Windows Update can pull out a previous version of your machine's programs and settings, then restore them properly.
Until yesterday, those old multigenerationally obsolete files just sat in the WinSxS folder. Now a new feature installed in KB 2852386 lets you delete a whole lot of festering garbage. On my workhorse Win7 machine, I got back 3.2GB (yes, gigabytes) of Windows bloat on my C: drive.
Behind the scenes, the WinSxS folder -- also known as the Windows' Side-by-Side component store -- holds every single version of every single operating system file that's ever graced your computer. WinSxS was introduced in Vista and lives on through Windows 8.1. Jeff Hughes explains on TechNet:
There is only one instance (or full data copy) of each version of each file in the OS, and that instance is located in the WinSxS folder. So looked at from that perspective, the WinSxS folder is really the entirety of the whole OS, referred to as a "flat" in down-level operating systems.
On most Windows 7 machines, the WinSxS folder gets huge. I have one Win7 machine with 8GB of files in WinSxS and another with more than 10GB. Unfortunately, you can't just delete the folder, as many of the items in the folder can be crucial to getting your system back up and running if a system file gets corrupted or compromised, or you roll back a Windows patch.
The WinSxS folder isn't supposed to bloat like a C: drive puffer fish, but it does. When you installed Win7 Service Pack 1, the installer included the option to delete all of the old, obsolete WinSxS files, and if you chose to cut the bloat then, you only have detritus from that point forward. The Windows Update Cleanup routine (as it's called) only runs as part of the Windows 7 Service Pack installer. Windows 7 hasn't had a Service Pack since March 2011, and in the interim all of those generations-old backups have accumulated in the WinSxS folder.
Right now, in Windows 7, if you right-click on your C: drive, choose Properties, then click or tap the Disk Cleanup button, you see a summary of what files Windows can clean up -- temporary files, Recycle bin, log files, and such. If you then click or tap the Clean up System Files button, you see a more detailed list that includes various kinds of system files -- thumbnails, archived error reports, memory dumps. On most machines, aside from the Recycle Bin and various temporary files, the amount of reclaimable space listed in the old Cleanup dialog generally isn't much.
If you install KB 2852386, the situation changes dramatically. At least it did on my well-worn Win7 SP1 machines. When you click the Clean up System Files button, Windows adds a new category called Windows Update Cleanup, which involves removing all of the very old entries in WinSxS. On the in-the-wild Win7 SP1 machines I tried, I found 1.2GB to 3.2GB of wasted space just waiting to be cleaned up.
N.B.: The new feature/setting only works on Windows 7 SP1 machines. Windows 8 (and 8.1) already have the capability built into the C: drive's Properties/Disk Cleanup dialog, and Windows XP doesn't work the same way. While Vista has a WinSxS folder, KB 2852386 isn't available on Vista machines, presumably because it hasn't been tested or doesn't work on the older OS.
If you have Windows Automatic Update set to automatically install updates (which I don't recommend) and to "Give me recommended updates the same way I receive important updates," you may have KB 2852386 installed already. If it isn't installed, and you want to try it, either go into Windows Update (Control Panel, System and Security, Windows Update, Check for Updates) and check the box next to the "recommended" update KB 2852386, or go to the Knowledge Base article and download it from there. Installation goes quickly, no reboot required. After it's installed, right-click on your C: drive, choose Properties, Disk Cleanup, then Clean up System Files. Look for the entry marked Windows Update Cleanup.
Although this new feature is still obscure, I haven't seen any reported problems and don't expect to see any. KB 2852386 appears to be a repackaging of the Service Pack 1 installer routine, and I haven't heard of any problems with that version.
While Microsoft hasn't released a simple automated Windows Update Cleanup admin program, Charity Shelbourne on TechNet has published detailed instructions on how to set up and push a client cleanup on a periodic basis. Yes, you should be able to reclaim 1GB to 3GB -- or more -- of C: drive on all of your Win7 SP1 machines. Not bad for a few hours' work.
Tip o' the hat to the folks at Microsoft who are still working on Windows 7. We appreciate your efforts, folks!