Hey all, never built a computer before but I recently bought a PC off a friend and I've had a hell of a time trying to get it to work. Brief background - I know this PC is maybe 5-7 years old, after my friend sold it to me I realized the Windows 10 installation was deeply screwed - the thing wouldn't even reset properly so no way to reinstall/wipe the drive, plus shortly after I started manually reinstalling programs (Steam in particular) the screen started flashing intermittently once I loggd in and wouldn't let any programs open.

I figure, fine, I'll just get a new hard drive altogether and reinstall Windows. The original drive is a Seagate Barracuda HDD; I got a Samsung 860 EVO SSD to replace it (the motherboard is an ASRock Z77 Extreme4, if that helps).

For some reason, the PC won't recognize or boot up with the replacement SSD plugged in, nor any other internal hard drive (I also tried with a friend's old HDD to make sure). The machine itself seems to boot up just fine, USB drives light up, etc - but no signal on the monitor. I tried switching SATA ports and nada. Tried booting up the computer with both the old and new drive plugged in and it boots into the Seagate drive, which doesn't help me much considering aforementioned Windows fuckery.

Any suggestions? Is this thing just kind of a lost cause? I've been bouncing around various forums and can't figure out a reason why the machine wouldn't be able to boot with the new drive installed. Kind of at a loss here.