A couple of points:
1. You don't mean "an OS registration problem," you mean an OS
*activation* problem. It's activation that's required. Registration is
entirely optional, and of no use to you. It's used by Microsoft for
marketing purposes.
2. The biggest restriction of an OEM version (which you have) is that
it is permanently tied to the first computer it's installed on, and
may never be moved to another. It's never clearly defined what
constitutes the same computer, and you will likely get responses from
some people who will say that changing the motherboard makes it a
different computer.
However, they are not correct. For a long time, it wasn't clear
exactly what constituted the original computer, and many people felt
that replacing the motherboard made it a different computer.
However, Microsoft has clarified the situation. See
http://www.microsoft.com/windows/products/windowsvista/buyorupgrade/activationfaq.mspx
or
http://tinyurl.com/384gx5
which states
"If you acquired Windows Vista pre-installed on a computer from a
major manufacturer (sometimes referred to as an Original Equipment
Manufacturer or OEM), Windows Vista will require re-activation if you
replace the motherboard with a motherboard not provided by the OEM."
So clearly, if you can reactivate it, it's legal to use it.
Although that page is specifically about Vista, it's perfectly
reasonable to assume that the same thing applies to XP.
3. Despite what it seems, the motherboard on eBay may or may not
actually be identical to the original one. Worst case, you may have to
reactivate Windows, but that should be no problem.