However, once built, the boot image can be simply copied onto devices, patched within reasonable limits, and remains disposable in case of any problems (viruses in particular). This is possible because unlike other hard drive images (which may contain any data, et al.), pure boot images contain no mission-critical data. By definition a pure boot image contains no data that cannot be reproduced from configurations or off-the-shelf executables. In particular end-user data is not part of a boot image, although some operating systems require that a copy of user preferences or configuration files be kept within the boot image itself, e.g. Microsoft Windows registry. Utilities like Norton Ghost keep a backup copy of the boot image, for quick re-imaging (often called re-installation) in the event of a problem, thus avoiding the need to diagnose a specific problem with a specific machine.