Boot from the recovery DVD and try to repair the operating system. If that didn't fix it, you need to reset the hard drive to factory defaults but you will lose everything that is stored in drive C.

1. Turn on or reboot/restart the computer.
2. When the VAIO logo appears on the computer's screen, briefly press the F10 key on the computer keyboard. The VAIO Recovery Wizard appears. Note that depending on the model of Sony Vaio, you may have to press either the F11, F12 or the ESC key to enter this restore mode. This must be done before your operating system begins to load. If you see the Windows logo appear on screen, you'll have to restart and try again.
3. Choose the option on screen that is most appropriate to your needs. You will be presented with three options: you can either create a set of recovery discs, attempt to repair your operating system's installation, or restore your operating system. Repairing your operating system will attempt to fix your computer without deleting any of your personal files. Restoring your operating system will delete everything on your hard drive and return your computer to its original factory settings.
4. Wait for the restore process to complete. Depending on the type of restore you've tried, you may have to wait as little as a few minutes and as much as an hour or more. When the process is finished, the computer will automatically restart itself.

Go to this link if you still have a problem:
http://www.kb.sony.com/selfservice/micro…

Note: If you want to back up your data before doing a reset, boot in Mini Windows XP using this bootable CD:
http://www.hirensbootcd.org/download/

View the original article here