It's not a file that's missing, it's the Windows Registry that's corrupted. The Registry is created when Windows is installed, you can't download it from anywhere. It must be repaired or Windows must be re-installed, or perform a factory recovery on your netbook:
For repair procedure read this: http://support.microsoft.com/kb/307545
To perform a factory recovery requires that you hold down a certain key or a combination of keys when you switch it on. Read the user guide for more instructions or go to your netbook manufacturer's website and look for recovery instructions for your model: HP Support: http://www8.hp.com/uk/en/support-dr...