Recently, on a laptop that I've used for a while, I had a strange issue where OS files were corrupted (device manager) and Windows 8 was hung after the login screen, so I reinstalled Windows 7 over the existing Windows 8 installation, and it worked for a couple days. Today, when I tried to use my laptop, it was stuck on a boot loop. Right after the BIOS screen, it would show a flashing underscore, then restart the computer, again and again until I removed the battery. I tried booting to a windows 7 install CD, but the same flashing underscore -> reboot sequence happened when I tried. I tried moving the boot priority around (HDD first, CD/DVD first, even USB first) but nothing changed. After about an hour of tinkering with it, I listened to the HDD sounds, and it sounded like the HDD was trying to spin up, but failing (whining noise increasing in frequency that stopped and started in sync with the system restarting). I am planning to replace the HDD, but I'm still confused as to why a faulty HDD would stop the laptop from booting to my install DVD (tried it on a different computer, it booted from that CD fine). Anybody here have any idea why this might be happening?