Manually deleting the Windows.old folder may corrupt the harmony of the OS. Do it with Disk Cleanup. Open the disk cleanup. Choose “Cleanup system files” option. You will be able to get rid of the following.
- Old windows backup
- Temporary installation files
- Upgrade logs
Ref: http://windows.microsoft.com/en-in/windows7/how-do-i-remove-the-windows-old-folder
This is a link for windows7, still it works in windows8 as well
