In the Control Panel, Administrative Tools, there is a Disk Cleanup utility.
Start that and select the proper drive. Then go to the bottom and select "Cleanup System Files". It will ask you to reconfirm the drive and look for those files. When it comes back, go through the check boxes and look near the bottom for Install type files. I do not have a Windows.old folder, so I cannot tell you the exact terminology.
If you want to check what files are included, select that entry, then the view files button. The Windows.old folder should be there, possibly with others also. Just make sure and uncheck any boxes that contain files you do not want removed.