If it was an OEM computer, it would be on the COA sticker except Windows 8 no longer does this. If it's OEM, then it's probably Windows 8 64bit, not PRO.
I have not tried the following yet so I can't confirm if it will work.
What you can do is install Windows 7 to the new hard drive as a temporary measure and then use the following method to pull the Windows 8 key from BIOS.
If you do not want to install WIndows 7 use a live CD of linux , Parted Magic should work, and try the following or this
Then use this utility to determine what version of WIndows 8 you have. Or use the online PID checker here.
You can use the recovered key to see if you can install using your iso files you downloaded. You may have to do a phone activation. Use the get_win8key script to find the key in firmware.
Edited by JohnC_21, 27 October 2014 - 01:11 PM.