Upgrade and downgrade issues - Dell inspiron 15-3521

Hi, I'm not sure if this is the appropriate section for this, but I'm working on a friend's Dell inspiron 15-3521 that has been unusable for about a month. The laptop is about 3 years old and came with Windows 8 and he tried to upgrade to Windows 10 when prompted. After the "upgrade", the laptop would boot correctly into Windows 10 about 1 out of 5 attempts and when it did boot correctly, it would ultimately crash without much explanation. He wanted to downgrade all the way to Windows 7(which I have a legitimate copy of) so I booted into a Linux Mint Live distro, formatted the drive, and installed Windows 7. After the installation, Windows 7 booted up fine but was incredibly slow and still basically unusable. Now, after restarting, the computer either restarts at the login screen of Windows 7 or freezes on a black screen after entering login info. Initially I thought maybe the ram was bad and was just running out of memory, but the Linux memtest comes back clean. Now I suspect that it could be a bad hard drive, but it would be great to get more opinions before I continue guessing.