Boot failure

My problem began a couple of days ago when my PC desk-top froze and wouldn’t respond to the mouse or keyboard. I tried CtrlAltDel but with no response so I switched off. When I re-started the PC it wouldn’t boot up. Startup repair was unable to repair the problem automatically and the system diagnostics reported: “Unknown Bugcheck 6b… Repair Action: System files integrity check and repair. Result: Failed. Error Code = 0x45d”.

I went into the BIOS and tried booting up from an Alienware repair disc I had made a year or so ago. This loaded the Alien Respawn Emergency file and folder backup utility. The utility has now been running for more than 24 hours(!) and I suspect it is not working. I have considered switching off and trying to boot from the original Alienware disc supplied with the PC but I am fearful of damaging the hard drive. Suggestions please!