It is possible that a back-up of the operating system was installed in a separate partition of the hard drive. Turn your computer on and straight away start to slowly tap the F8 key (if this does nothing, try F1). After a few moments the Advanced Boot options menu should appear. Select the Repair Your Computer option. Choose the language that is applicable to the country you live in. On the next screen choose 'Start Microsoft Windows Repair Environment. Your computer will now attempt to repair any errors and when completed you will be asked to restart your computer.