This folder contains the contents from the previously installed Operating System, and can safely be removed if you haven’t placed anything in the Windows directory that you need to retrieve.
Here is how you remove it:
1. Open Windows Explorer
2. Right-click C: and select Properties
3. Select Disk Cleanup
4. In order for you to be able to remove the Windows.old folder, you need to select Clean up system files. Because this is an elevated task, you might receive a UAC prompt. Press Yes at the prompt.