Hi teewhy. You haven't told us what the motherboard or CPU are, so it's difficult to give a specific answer. Whoever it was that told you that the CPU, memory etc were OK was really rather jumping to conclusions, as without testing the parts in an otherwise good setup it's virtually impossible to be certain. I don't know how much you've spent so far, but in a situation like this, I would be inclined to buy a motherboard bundle from Novatech. This consists of a motherboard, CPU and memory all assembled and tested. click here.
If your Windows installation was oem, you might have problems with the installation because oem Windows is locked to the computer it was installed on, and with a new motherboard and CPU you will effectively have a new computer. If you phone Microsoft they might look sympathetically on you and revalidate your Windows. The other problem you might find is that some Windows installations are BIOS locked, and will only work with the computer manufacturers BIOS.