Install the OS on another hard-drive, then connect the first one as secondary, and all the data will be there. If you don't have another hard-drive, you can use a live Linux disk (Ubuntu), just try it (don't install it), and you can mount your hard-drive and then transfer your data, to another hard-drive (USB key or something), or online.
This is the safest way. You can try repairing the BootMGR, or try to re-install and repair on the disk, but something could go wrong and you might lose data.