wrt OEM, the terminology of a "new machine" is a gray area wrt JUST replacing mobo and CPU but using the same case.
Many of us have the belief that this is acceptable. AND that your OEM copy is not a "branded copy" (customized for Gateway, HP, Dell, etc.) . . .
Given that position.
If you are using the same case, etc. you can put the new mobo/CPU in, BOOT off of the OEM CD directly and run a REPAIR INSTALL of XP. This will preserve all your applications and most of your settings. After that, you'll be good to go with minimal headaches.
"A man who trims himself to suit everybody will soon whittle himself away." - Chinese fortune cookie (but politicians should take note!)