After installing Windows 10, Roy A. Day discovered a very large Windows.old folder on his internal drive. Since deleting it the conventional way didn’t work, he asked for another solution.

The Windows 10 upgrade (whichso many are enthusiastically embracing despite my warnings) leaves a huge Windows.old folder on your C: drive. On one computer I updated, it was over 25GB. And Windows won’t let you simply delete it.

There’s a very good reason why you shouldn’t. Without this folder, you will not be able to go back to Windows 7 or 8.1. Therefore, you should only remove Windows.old if one of these three situations applies to you:

• You’re absolutely sure you want to stay with Windows 10.
• You created an image backup before the upgrade, and can therefore restore your previous installation without Windows 10’s built-in tools.
• It’s been more than 30 days since you made the upgrade, and Windows 10 will no longer allow you to go back.

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If you meet one of those conditions, here’s how to free up a lot of room on your C: drive:

1.   In File Explorer, right-click C: (under This PC) and select Properties.

2.   On the General tab, click the Disk Cleanup button.

3.   Wait while Windows thinks about it. Eventually, the Disk Cleanup dialog box will appear.

4.   Click the Clean up system files button near the bottom of the dialog box.

5.   Wait some more. The dialog box will disappear, then a slightly different version will reappear, without the button that you just clicked.

6.   Scroll down the 'Files to delete' list until you find Previous Windows installation(s). Check that option.

7.   Check or don’t check other options. You’ll have to make those decisions yourself.

8.   Click OK. You’ll be asked twice to verify your decision. Answer in the affirmative.

You’ll have to wait a bit while Windows removes all those files. But when it’s done, you’ll have a lot more space.