Where did you learn about this?
I upgrade on 12-4-2012, and the Windows.old file is still on my system.
Regardless, you need to copy the files from this Windows.old folder to a good location. You need to do this for ALL of the files that you are using and/or want to keep on your system.
The biggest problem, with using the files from within this folder, if you ever need to reinstall the current OS or upgrade to another, this Windows.old folder will be over written by a new copy.