If you want to delete the Windows.old, open Disk Cleanup and select clean system files, it should give you the option to clean up the Windows.old folder, or you need to take ownership of the files to delete them.
If you run CCleaner and select the Windows.old file in the list it will remove it.
I think it appears in the "System" or "Advanced" category if it is present on your system.
If you don't already have CCleaner you can get it here...
I'd recommend that you have it installed in any case, I'm guessing that virtually everyone here uses it.
I've used it for many years and run it every day to keep all the junk out of my computer.