How to remove the Windows.old folder after Windows upgrade?

Suppose we have upgrade Windows to a newer Windows version. Then, there will be a folder under C drive called "Windows.old". Do we need to keep the Windows.old folder? What is it's purpose? Can we delete or remove the Windows.old folder? How to remove the Windows.old folder after Windows upgrade?

What is the Windows.old folder? 

If we install Windows 7 by performing a custom installation without formatting the partition during the installation process, files that were used in the previous version of Windows are stored in the Windows.old folder. 

Can we delete or remove the Windows.old folder? 

After using the new Windows for a while, and we are surly set, confident that your files and settings are back to where we want them to be, we can safely reclaim disk space by using Disk Cleanup to delete the Windows.old folder.

How to remove the Windows.old folder after Windows upgrade?

• Press Start button
• In the search box, type Disk Cleanup, and then, in the list of results, click Disk Cleanup.
• If you're prompted to choose a drive, click the drive you just installed Windows on, which normally is the C drive, and then click OK.
• In the Disk Cleanup dialog box, on the Disk Cleanup tab, click Clean up system files.
• If you're again prompted to choose a drive, click the drive you just installed Windows on, and then click OK.
• Select the Previous Windows installation(s) check box, and any other check boxes for the files you want to delete, and then click OK.
• In the message that appears, click Delete Files.

 [Source: Microsoft]