|#2 is correct. You will at least need to do a repair install and then change your drivers if the boards are similar, but most of the time (different chipset) you will need to reinstall your operating system completely. If this is a factory built system you may need to use a retail install disk and a new Windows 7 key to reinstall.|
Your BIOS is on your motherboard, NOT on your hard drive. BIOS is Basic Input Output System and needs to load first before you can load your operating system before your system can read any drive. You appear to be new to this so I will not assume automatically you have installed everything correctly. Unplug your hard drive and anything not essential and remove all but one stick of memory and then boot into your BIOS set up, reset defaults and then check your internal temperatures, If your CPU or other parts are running HOT or the temperature is climbing fast (or non stop), exit and shut down the power. You will need to reassemble it correctly. If you cannot get into your BIOS then strip it down, check components for compatibility, then bench test it outside of your case with JUST the basics. Google 'Bench Test Computer' for details on the correct way.
Whenever you remove the heat sink from the CPU you need to clean off the old thermal compound and replace it with a small amount (applied the correct way for your CPU) of new thermal compound before reattaching them on the new board. Look:
Post back details and more information for more detailed help. Don't assume anything, if you do not KNOW, ask. Google is a useful tool (use it) but sometimes you need to know what you need to ask.
You have to be a little bit crazy to keep you from going insane.