It's one of the most common things we hear: "My computer says the drive is nearly full, but I haven't saved anything new in months." The customer looks baffled, and honestly, you can't blame them. When you open File Explorer and start adding up the obvious folders — Documents, Pictures, Downloads — the numbers just don't add up. There's clearly something else in there, lurking.

Nine times out of ten, the culprit is a folder most people have never heard of. And the good news is that Windows will often let you clear it out in about three clicks.

Meet the WinSxS Folder — Windows' Hidden Hoarder

Deep inside your C drive sits a folder called WinSxS (short for Windows Side-by-Side). Its job is to keep copies of Windows system files — including older versions — so that updates can be rolled back if something goes wrong. In theory, that's sensible. In practice, it can quietly balloon to 10, 20, even 30 gigabytes on a machine that hasn't been properly maintained.

If you navigate to C:\Windows\WinSxS and check its size, you may have a small shock. Don't panic — and don't just start deleting things inside it manually. That's a reliable way to break Windows entirely. But there is a safe, built-in way to trim it down.

The Safe Way to Reclaim the Space

Windows includes a tool called Disk Cleanup that handles this properly. Here's what to do:

  1. Click the Start button and type Disk Cleanup, then open it.
  2. Select your C drive and click OK. It'll spend a moment calculating.
  3. When the results appear, click "Clean up system files" at the bottom — this is the important step that most people miss. It'll scan again.
  4. Look through the list for items like Windows Update Cleanup, Previous Windows Installations, and Temporary Files. Tick everything you're comfortable removing.
  5. Click OK, confirm, and let it run.

It isn't unusual for this process to free up 5–15 GB on a PC that's been running for a couple of years without a clean-up. On older machines we see in the shop, we've occasionally recovered 25 GB or more in a single pass.

The Other Usual Suspects

WinSxS gets the headlines, but it rarely acts alone. These folders are worth checking too:

  • C:\Windows\Temp — Temporary files that Windows creates and frequently forgets to delete. You can clear the contents of this folder safely (leave the folder itself).
  • C:\Users\YourName\AppData\Local\Temp — Your personal temp folder. Same deal — contents can go, but don't delete the folder.
  • The Downloads folder — Worth a look. It's remarkable how many gigabytes of forgotten installer files, PDFs, and video clips accumulate here over a few years.
  • The Recycle Bin — Easy to forget. Right-click it on the desktop and choose Empty Recycle Bin.
  • Hibernate file (hiberfil.sys) — If you never use hibernate mode, this hidden file can occupy several gigabytes. It can be disabled via Command Prompt, though that's a step worth Googling carefully or asking someone to help with.

What About Storage Sense?

If you're running Windows 10 or 11, there's a newer, more hands-off option built into Settings. Search for Storage Sense in the Start menu and switch it on. Once enabled, it periodically clears temporary files and empties the Recycle Bin automatically, so this kind of build-up is less likely to sneak up on you.

It won't do everything Disk Cleanup does — particularly around Windows Update leftovers — but it's a decent safety net between manual clean-ups.

When a Clean-Up Isn't Enough

Sometimes you do all of this and the drive is still uncomfortably full. At that point, the issue usually isn't clutter — it's capacity. A lot of laptops and budget desktops shipped with a 128 GB or 256 GB SSD, which seemed plenty at the time but fills up faster than most people expect, especially once Windows updates, photos, and applications start competing for space.

Upgrading to a larger SSD is often cheaper than people assume. A 500 GB or 1 TB SSD upgrade typically costs well under £100 including fitting, and the difference in day-to-day speed and breathing room is immediately noticeable. It's one of the most worthwhile things you can do for an ageing PC or laptop — far more so than buying a new machine because the old one "feels slow."

That sluggishness, by the way, often has as much to do with a full drive as with anything else. Windows needs free space to work properly — as a rule of thumb, try to keep at least 10–15% of your drive free at all times. Drop below that and things start to drag noticeably. A slow PC that's simply run out of room is one of the most satisfying fixes there is.

The Practical Takeaway

If your PC is telling you the disk is nearly full and you can't see why, run Disk Cleanup — using the "Clean up system files" option — before you do anything else. It's free, it's built in, and it takes about five minutes. Then enable Storage Sense so the problem is less likely to creep back. If the drive is genuinely just too small for your needs, an SSD upgrade is almost always the better answer over buying a replacement computer.

A little housekeeping goes a long way, and you don't need to be remotely technical to do it.