Did you not perform a repair installation when you installed the new
motherboard?  If not, you'll probably have other problems, as well.

    Normally, and assuming a retail license (many factory-installed OEM
installations are BIOS-locked to a specific chipset and therefore are
*not* transferable to a new motherboard - check yours before starting),
unless  the new motherboard is virtually identical (same chipset, same
IDE controllers, same BIOS version, etc.) to the one on which the WinXP
installation was originally performed, you'll need to perform a repair
(a.k.a. in-place upgrade) installation, at the very least:

How to Perform an In-Place Upgrade of Windows XP
http://support.microsoft.com/directory/article.asp?ID=KB;EN-US;Q315341

Changing a Motherboard or Moving a Hard Drive with WinXP Installed
http://www.michaelstevenstech.com/moving_xp.html

    The "why" is quite simple, really, and has nothing to do with
licensing issues, per se; it's a purely technical matter, at this point.
 You've pulled the proverbial hardware rug out from under the OS.  (If
you don't like -- or get -- the rug analogy, think of it as picking up a
Cape Cod style home and then setting it down onto a Ranch style
foundation.  It just isn't going to fit.)  WinXP, like Win2K before it,
is not nearly as "promiscuous" as Win9x when it comes to accepting any
old hardware configuration you throw at it.  On installation it
"tailors" itself to the specific hardware found.  This is one of the
reasons that the entire WinNT/2K/XP OS family is so much more stable
than the Win9x group.

    As always when undertaking such a significant change, back up any
important data before starting.