|"Why won't it work?"|
Because Microsoft has designed Windows to detect hardware changes, especially the motherboard. If you attempt to use the old Windows installation on your new system, the difference in hardware will be detected & Windows will refuse to boot. Microsoft deliberately did that to prevent hard drives from being transferred from one system to another without doing a reinstall. And since you're "legally" not supposed to use the same product key more than once, they're "forcing" you to pay for another copy of Windows. You can however, install Windows over the top of itself. This was called a "repair installation" for WinXP, for Win7 it's called an "in-place upgrade installation".
"What would u reccommend for a computer thats below $700? I dont need a graphics card nor dvd drives. But, has to be a full tower case"
Why does it HAVE to be a full tower? They're generally much more expensive than a mid-tower. Are you installing 4-5 hard drives & several optical drives? Or do you think it's going to keep things cooler? Case with bottom mount power supplies are NOT the way to go. With the PSU at the bottom, it no longer helps to cool the case, plus the incoming air is restricted by the perforations in the case, not to mention it's only sits about a 1/2" off the floor or table. It's really a poor design. But people buy em, so manufacturers keep making them. Cases with top mounted PSUs are superior.
The Problem of Bottom-Mounted Power
You really didn't answer my followup questions about TigerD & the RAM? Here's a configuration to consider:
COOLER MASTER ELITE 335 Mid Tower case- $50
CORSAIR Enthusiast Series CMPSU-650TX 650W PSU - $80
ASRock 870 EXTREME3 AM3 AMD 870 motherboard - $90
AMD Phenom II X6 1090T Black Edition - $200
G.SKILL Ripjaws Series 4GB (2 x 2GB) DDR3-1600 - $50
Total = $470