The Bitdefender Rescue CD is a tool that will attempt to repair Windows if it will no longer reboot, for example as a result of BitDefender incorrectly quarantining vital system files.
The Rescue CD comes in the form of an ISO disc image, and so you'll need to use your disc burning software to create a physical copy of this, first. Not sure what to do? Then you can alternatively use a specialist image burning tool to help you out: ISO Workshop, as an example, will do the job for no charge at all.
Once you have a disc then all you need to do is boot your affected PC from it, and follow the instructions. There's nothing complicated involved, essentially the Rescue CD will scan your system and automatically repair any issues it finds.
Please note, Bitdefender point out that this tool only works on "English, French, Spanish and German versions of Microsoft Windows 8, Windows 7, Windows Vista, and XP (for XP only the version that is installed on the C drive)."
And in addition, it only applies to the following Bitdefender products from 2010 to 2014. You need one of these installed to use the Rescue CD.
The Bitdefender Rescue CD is an easy-to-use tool which might get an unbootable system working again. It has several limitations, though, so we'd use all the regular Windows repair options first - this should very much be a last resort