Yesterday I also exchanged the RAM and the battery, but I put the old ones back in and it still behaves the same so I don't see how that could be the issue.

I have another fresh hard drive that I could swap, but when I did swap it and then turned the computer on, it just goes to the apple and the spinning circle and stays there forever.

Try running this application again.' That's as far as I can get, and to clarify, I haven't been able to get beyond OS X Utilities at all. Soon into the installation process, after it asks to verify my information with apple, a message pops up: 'An error occurred while preparing the installation. When I go to 'Reinstall OS X' it launches right into the Capitan installation, not the OS X 10.6 installation CD which is in the drive. When I go to Disk Utility I see Capitan there, but as with my hard drive, I can't erase it and neither can I eject it. Now here is what seems to have triggered the issue: I had just downloaded OS X Capitan and was attempting to install it, but the installation failed after the part where the computer restarts. I tried erasing the hard drive (I have a recent backup), but it always tells me 'Erase process failed.' I tried restoring from Time Machine Backup, and although it found my backups on an external hard drive, it doesn't seem to recognize them. I tried repairing the hard drive in Disk Utility with first aid, but it says it can't complete the repair. After 5-10 minutes, it asks me to select a language, and then proceeds to OS X Utilities. Upon pressing the power button, the computer immediately turns on and chimes like normal, then goes to the white screen with the apple and a progress bar. I have tried safe mode, resetting PRAM and SMC, recovery mode, everything I know of. My early 2011 Macbook Pro which uses 10.6 Snow Leopard is going straight to OS X Utilities upon startup, no matter whether I use any startup keys or what those startup keys are.

Mac os x utilities screen