so..you bought a new motherboard..and installed it..hooked up the old hd with the old install of xp on it and tried to run it? right?
you need to try a repair install of xp on the drive...hopefully you've only tried to boot it once...xp requires being installed on one hardware platform...so...trying to run it on a new platform means all the drivers and irq's are wrong....IF the repair option doesn't work...your only option is to do a fresh install of the os
Boot up with the XP CD in the drive and let Setup run. Go past the "C" and "R" repair options by presssing Enter just as if you're doing a clean install and you'll eventually get an option to "Repair an existing installation". That's the one you want. If it asks about formatting you've gone wrong, so start over.
You'll get to keep your existing apps & settings, but you need to re-install SP2, Windows Updates & any unsigned drivers afterwards. NOTE: Be aware that you'll lose any protection from Blaster / Sasser type worms. Don't go online without a firewall, whether the built in Windows one or 3rd party.How to Perform A Re-Install Of XPRepair XP