It all depends... if you get a popular brand comp it is sometimes near impossible to load a generic Windows copy on it. Although these usually come with an operating system installed and an opportunity to make a copy (once).
If you know someone who can install part (the kid two doors down?) and you're happy with your Windows OS and the desktop box, why not just upgrade the parts. A new motherboard, CPU and RAM is a lot cheaper than a made up comp. You can do a deal that you might get a new harddrive for half price etc.
Make sure the new motherboard is compatible with your Windows version, it should say so on the box.