Black Screen with flashing underscore in top left of screen upon startup

Hello, I recently bought a used computer (HP 8100 elite) from my polytechnic college that came with no RAM or HDD for dirt cheap. I bought a WD red 2TB HDD and 4GB Kingston ValueRam PC3-10600 DDR3 CL9 DIMM. My plan for this PC is to run Windows Server 2012 R2. I first got Windows Server 2012 R2 on a bootable USB. Then when I power on the PC it will start the loading files process bar. When the loading bar reaches the end the Windows logo appears. Then the PC restarts and starts the loading files progress bar then Windows logo appears and the PC will restart again and do the same thing over and over again. So.. I put the new hard drive into my desktop and installed Windows Server on that, and it works as expected. Then I put the hard drive (with windows server installed) into the used PC. When turning on the PC and entering the BIOS the computer recognizes the RAM and the HDD. But when leaving the BIOS and letting the PC boot I get stuck at a black screen with a flashing underscore in the top left of the screen.

I have also tried putting my SSD (Dual booted with Linux and Windows 10) that I use for my desktop into the used computer and i still get the same black screen with flashing underscore in the top left of the screen.

I have tried reseating the RAM and trying different slots but to no avail. I tried using different SATA ports but that doesn't change anything.

If anyone could help me, that would be greatly appreciated. Thank you

PC specs 2010 HP 8100 elite -convertable mini tower Intel Core i5 650 3.2 GH Dual Core WD red 2TB HDD Kingston ValueRAM 4GB PC3-10600 DDR3 CL9 DIMM