That problem is one that is just painful to get, however as we can tell the first priority is to back up your data.
If you have all you data backed up I would recommend just using either Vista installer to reinstall Vista, or the Recovery Discs to get your OS back.
If you do not have your information backed up I suggest you do so right now using this method
This is a step by step tutorial that will guide you through this process.
If you need any help please do not hesitate to post with questions.