How do I open a .old file?

Rename it back to its real extension (remove the .old), or open a copy in the app that made it.

Can I delete Windows.old?

Yes. Use Disk Cleanup and tick 'Previous Windows installation(s)'. You just lose the ability to roll back the upgrade.

What made the .old file?

Usually an installer, driver update, or edit that renamed the original before replacing it.

What Is a .OLD File?

You spotted a file ending in .old, like hosts.old or Windows.old. It is a saved copy of the original before something replaced it. Here is how to restore it or clear it.

What a .old file is

A .old extension is not a real format — it is a renamed backup. When an installer, driver update, or edit is about to overwrite a file, it often renames the existing one by adding .old so the previous version can be brought back if the new one misbehaves. Examples: hosts.old, a driver's .old, or the giant Windows.old folder left after a Windows upgrade.

How to restore it

Because the real format is hidden under .old, you restore it by renaming: delete or move the current file, then remove the .old from the backup (e.g. hosts.old → hosts). The file works exactly as before.

Is it safe to delete?

Yes, once you are happy with the replacement. A .old file or the Windows.old folder is only a fallback. Remove Windows.old with Disk Cleanup (it can reclaim many gigabytes) rather than deleting it manually, since parts are protected.