In the event of a serious error on your hard disk or PC which may prevent Windows from loading, it is a good idea to have a bootable Windows Operating CD or DVD available so that you can boot your system and troubleshoot the error. This can be done using either your original Windows installation discs, or media provided by your computer manufacturer for system recovery. However, if you do not have either of these you can create a system repair disc from within any installation of Windows Vista SP1 or Windows 7 which you can keep on your shelf to use in an emergency. Read more: Microsoft Help Blog