When booting a computer with a Windows OS, sometimes there is an error that reads “Unmountable Boot Volume” and that error message is giving you an early warning that your hard drive is failing. To confirm if this is so, go into the BIOS and enable SMART Reporting (Self-Monitoring, Analysis and Reporting Technology) so you can confirm it during POST.
Should the hard drive completely fail to make it to the login prompt or boot, make that failing hard drive a slave drive inside your computer then copy the data to a replacement drive that has your OS configured. If your computer does not support adding a second hard drive, temporarily removing the optical drive is one option of doing so. If you add the hard drive and the OS sees that it is hidden, unhide the hard drive and continue.
Please keep in mind the the optical drive and the hard drive has to be the same IDE or SATA factor for this swap to work. If not, borrow or buy a USB to Hard Drive adapter that will do the same job for you.