The change of motherboard is preventing it from loading. The best you can do is repair XP but you will have to install all the updates that are missing from the WinXP CD, e.g. SP2 plus about 20-30 since SP2.
To repair XP. Boot from the WinXP CD (in the BIOS boot sequence put the CD drive ahead of the HDD). When the first message comes up, choose “To set up WinXP” not “R for recovery console”. Windows will eventually recognise that XP is installed and ask what you want to do, press R to repair and follow the instructions except after rebooting don’t reply to the message “Press any key to boot from CD”, just wait.