First, your Windows won't work as soon as you put in a new motherboard, it is tied to the hardware. 2) There is no compatible or salvageable parts from a 'elderly' computer except the HDD. So using it is really not the 'best / least cost' solution. PCs are now 'disposable' like cell phones and as similiarly priced. Depending on what your going to use this for, a simple 'PC' is just $249 at Walmart (i3core, Windows, 6-8GB RAM, 500GB+ drive, DVD, and alot of times free LCD, mouse keyboard all in one box). For more 'robust' or gaming you could go $349 (i5Core, similiar stuff) then upgrade the PSU ($99+) to a 600W or so, and the GPU for whatever you can afford ($149-$500) for High level graphics on BF4 for example.