Legally speaking, if the OS was an OEM version, then it stays with the original MB and CPU. You need a new copy.
However, you should be able to just start from the Win 7 disc and only need the XP discc when prompted. Remember that MS is more picky about Win 7 and you'll probably have to jump thru hoops to revalidate the install
No you can just install a fresh copy with your new set up I have done it a million times you might have to call the automated activation service but maybe not if so just say you have installed your copy on one machine.