You need to go into BIOS to change the boot order from harddisk as no. 1 to CD as no 1. When you power on PC look quickly for any instruction on which key to press to go into BIOS setup. It is unusually F1, F2, ESC, DEL or F10. If you have never changed BIOS before then you may need to surf around to get some education. If your diagnostics says disk failure, I would not boot it any more, especially if the disk had been well used and do a repair on it. The repair disk activity will stress and strain an already old failed disk, the end result could be terminal. Without thinking much, you need to remove the disk and put it into caddy and try to retrieve important data from it and look at the SMART disk data to see what may be wrong..... The main thing is that you need to stop too many disk I/O activities. If the disk still works, concentrate to get the most important data out first.
Best Amazon Echo: What’s the best Alexa speaker?