You haven't downloaded anything new. You haven't installed anything you can remember. Yet somehow your hard drive — or even your SSD — keeps quietly filling up, a gigabyte here, a gigabyte there, until one morning Windows pops up that annoying "low disk space" warning and you genuinely don't know where it all went.
This is one of the most common things we see at our Tenterden repair bench. And more often than not, the culprit is a folder most people have never heard of: WinSxS.
What on Earth Is WinSxS?
Deep inside your C:\Windows folder — hidden away where most people never look — sits a folder called WinSxS (short for Windows Side-by-Side). On a healthy, well-used PC it routinely runs to 10, 15, even 20 gigabytes. On older machines we've seen it creep past 30 GB.
Windows uses it to store multiple versions of system files — old ones, new ones, backups of components that got updated. The idea is sensible enough: if a Windows Update breaks something, the system has the old version tucked away and can roll back. It's a safety net built into the operating system itself.
The problem is that Windows doesn't always clean up after itself very well. Every update potentially adds more files to the pile, and the folder just keeps growing. Windows does do some housekeeping automatically — but not always thoroughly, and not always quickly.
How to See How Big It Really Is (Without Panicking)
Before you do anything, it's worth finding out just how much space we're actually talking about. Here's how to check:
- Press the Windows key + R, type cleanmgr and hit Enter.
- Let Disk Cleanup scan your drive — this takes a minute or two.
- When it finishes, click "Clean up system files" (you may need to confirm an admin prompt).
- It will scan again, then show you a longer list. Look for "Windows Update Cleanup" and "Previous Windows installation(s)" — these are typically the biggest wins.
It's not unusual to find 5–12 GB recoverable here on a machine that's been running for a year or more without a proper clear-out.
The Right Way to Reclaim the Space
The most important rule: do not manually delete files from the WinSxS folder itself. You'll see advice online suggesting you dive in and start deleting. Don't. Windows actively manages that folder in the background, and some of what looks like a duplicate file is actually a hard link — delete it the wrong way and you can cause real damage.
Instead, use the tools Windows provides. They're a bit buried, but they work properly:
- Disk Cleanup (cleanmgr) — as above. Tick "Windows Update Cleanup" at minimum. This is safe, built-in, and free.
- DISM command — for those comfortable with a command prompt, running DISM /Online /Cleanup-Image /StartComponentCleanup as an administrator tells Windows to do a deeper tidy of the component store. It can recover space that Disk Cleanup misses.
- Storage Sense — in Settings → System → Storage, you can turn on Storage Sense to run cleanup tasks automatically on a schedule. It won't touch WinSxS directly, but it handles temporary files, the Recycle Bin, and old Downloads automatically.
One important caveat: after running Windows Update Cleanup, you can no longer uninstall the recent updates it removes the backups for. For most people on a stable, working machine, that's a perfectly fine trade-off. If your PC is behaving oddly after a recent update, wait a week or two before running the cleanup — just in case you need to roll back.
Is This Why My PC Is Running Slowly?
Disk space and performance are connected, but not always in the way people expect. A full drive — particularly an older spinning hard drive — will slow Windows down noticeably once it gets above about 85–90% full. Windows needs free space to write temporary files, manage virtual memory, and breathe in general.
On an SSD, the picture is slightly different. SSDs need free space to manage their internal housekeeping (a process called wear levelling). A very full SSD will slow down and, over time, wear out faster.
So yes — reclaiming even 8–10 GB from a bloated WinSxS store can make a real, tangible difference on a machine that's been feeling sluggish. It's not magic, but it's a surprisingly easy win that costs nothing.
If your drive is still feeling squeezed after a cleanup, it may be worth considering whether a larger SSD is the right next step. A storage upgrade is one of the most cost-effective ways to breathe new life into an older machine — and the difference in day-to-day speed is remarkable.
Other Hidden Space Hogs Worth Knowing About
While you're on a mission to reclaim space, here are a few other places worth checking:
- The Hibernation file (hiberfil.sys) — if you don't use hibernate mode, this file can take up several gigabytes. It's safe to disable if you prefer Sleep instead.
- The Temp folder — press Windows key + R, type %temp% and hit Enter. You can safely delete everything in that folder (some files may refuse if they're in use — just skip those).
- Old user profiles — if multiple people have logged into the same PC over the years, old unused accounts can leave large profile folders behind under C:\Users.
- Downloaded Windows installers — check your Downloads folder for large .exe or .iso files from software you installed months ago and no longer need.
None of this requires special software or technical expertise — just a methodical ten minutes with the tools Windows already gives you.
The Practical Takeaway
If your PC is running low on space and you haven't installed anything obviously large recently, WinSxS is almost certainly part of the story. Run Disk Cleanup as administrator, tick Windows Update Cleanup, let it finish, and restart. It won't fix everything — but it's the right first step, it's completely safe when done properly, and it costs absolutely nothing.
If you've done all of that and your drive is still mysteriously full — or your machine is still dragging its heels — it's worth getting someone to take a proper look. Sometimes the answer is a quick diagnostic; sometimes it's a storage upgrade that transforms a tired machine. Either way, it's rarely as dramatic as it first seems.