There are jumpers on the SATA drive to adjust certain built-in settings. I know on quite a few, in order to operate SATA drives that operate in both I and II, you have to move the jumpers, as you do on mine.. Sometimes having some whacky setting will just cause the HDD to operate wrong in both ways. It's good just to make sure that it is set correctly.
But that doesn't really matter. I don't think it's the CD drive either, since he can boot the disk when the HDD isn't plugged in. It's possible it's a problem between the two, but I have no idea what would cause that. Worst case scenario, he could find someone with a SATA-capable system, put his HDD in and try formatting and installing the OS on theirs, then porting it over onto his computer to see if it'll boot. If it won't work on the other system, or if he brings it back and it won't work on his own, then we've got some more insight into the problem. Unfortunately, hardware is one of the most annoying problems to try to treat, and trial and error is the only way to figure out the issue.
i downloaded a program called cpu-z to check some specs and when i executed it windows froze and a weird beep-like sound came from the speakers but nothing from the pc itself,
I'm trying to figure out what might have caused that, and what it in turn did to the computer. It's remotely possible something shorted it when it was gathering temperatures, fan-speeds, voltages, etc... But that'd be kinda rare -- not that rare things don't happen to computers often. Let's hope it's just something simple like the HDD being damage, and not the entire motherboard.