I have the greatest respect for Fo3nix but can I suggest a slight change to his/her answer?
Try running the Windows installation program without reformatting the drive. An 'on top' installation of Windows often corrects registry problems and avoids the need to re-format the drive. If it works, you'll keep all of your existing files and software, together with such things as your e-mail accounts and ISP log-on details.
If the 'on top' installation doesn't work, then you'll be forced to do a 'clean' installation after formatting the hard disk. However, this is a drastic step because it means that you'll lose everything on your hard drive, including documents, pictures, software and passwords.
Chris