|A warning to back up immediately usually means the hard drive is starting to fail, and eventually it will fail completely, taking all your data with it.|
If the hard drive has failed, booting from a disc to re-install Windows is futile. You need to buy a new hard drive first.
But before doing so, you may want to make quite sure that the existing drive has really failed, so test it with the drive manufacturer's diagnostic software for DOS. The links are on this page:
You need to download the DOS version which is an ISO file. With that, you can make a bootable CD that contains the testing software. Make the CD from the ISO file using IMGBurn from here:
When you've made the CD, boot your faulty PC from it to run the hard drive test.