Windows.old is where your previous files is located before the upgrade. I recommend checking things in there first as you might have some files that you want to copy over to the upgraded Windows/OS like pictures, games etc. After checking and copying over the files that you want, you can delete windows.old. Dont worry, it is not needed to run your computer as it was already replaced by the files in your root drive. Be sure your using an administrator account. How I know this? because Ive done this many times before, and The first thing I do is check for the files in the windows.old and copy what I want out in that location (windows.old) and delete it after I get everything I need. Hope this helps. Cheers