In some versions of Windows it is possible and in some it is not because Windows keeps some files open. If you are making a backup to replace system files if they become corrupt, a backup of the files that can be backed up should work. If you are making a backup to have to restore Windows if it crashes, it is better to make a clone of the entire drive. WinRescue does not make a clone.
In Windows XP, 2000, and Vista, the registry files are kept open and so cannot be copied but WinRescue works around that and does make a copy of the registry.