If you mean you've taken a W7 HDD from a previous PC and installed it on your W8 PC, then it's unlikely to boot unless the hardware of the old PC is identical to the new one.
If it came pre-installed on the old PC, it will have an OEM serial number and the EULA prohibits using it on any other hardware, so when it detects the change of environment it will not validate any attempt to re-activate it.
That being said, if you still want to attempt to boot it anyway, you'll need to change the BIOS boot sequence to select whichever HDD you wish to boot, How to access/enter Motherboard BIOS
or if you want to try creating a dual-boot, you'll need to use EasyBCD 2.2 and add an entry for W7 in the W8 BCD.
I'd try the former first to determine whether there's any point in going further.