You have paid for a Windows license for use with your PC, yes you have BUT, the PC you now want to install Windows on is not the one the license was for is it? A Windows XP license entitles installation in one machine. This has been done by the manufacturer. It does not entitle you to install on any other machines that take your fancy. That will come apparent as soon as you try to activate the XP version - a requirement of XP to be done within 30 days of first use.
Recovery disks (like you seem to have) rely on hidden information on the original computer hard drive and relate to the motherboard and attached hardware. If you have changed something as fundamental as the motherboard, this hidden information does not correspond to the new setup and thus will not work.
I can't understand why you "recently bought a brand new PC" and then "upgraded the motherboard". Why didn't you buy one with a motherboard that was suitable for your use?
Or have I misunderstood your whole post?