Windows.old folder saves the old version of your pc, so if you have updated your computer, you can old this folder.
How to delete this folder?
Here is proper way to delete this folder.
Step 1: Click in Windows’ search field, type Cleanup, then click Disk Cleanup.
Step 2: Click the “Clean up system files” button.
Step 3: Scroll down the list until you see “Previous Windows installation(s).”
Step 4: Click OK to start the cleanup.