|The 100% CPU usage has little to do with your motherboard or CPU or the fact that you added another HDD. My guess is your system is infected or just very poorly configured. You have an older system, that's true, but if it was setup correctly, it should run extremely well. For example, your problems with HD video is because you don't have a graphics card. The integrated Intel X4500 graphics is weak. A cheap video card would easily fix the 1080p problem. And if you were to overclock the CPU plus make sure the CPU & RAM run in sync, you would see a significant performance boost.|
Here's a Radeon HD 6450 with 1GB memory for just $20 after rebate:
On the software side of things, you should run a deep scan for both viruses & malware, use a cleaner such as CCleaner-Slim, manage your startup apps so they don't suck the life out of your system, make sure your the device drivers are up to date, & defrag your drives.
Switching the CPU & board will obviously cost you a fair amount of money, I'm guessing $200 or more, plus Windows 7 will have to be reinstalled due to the hardware change.