Presumably this is a new motherboard, in which case it is likely you will need to reinstall all motherboard component drivers. If you don't want to format and start all over again, you can do a repair reinstall, i.e. installing XP Home on top of itself without formatting.
Before you change motherboard, click here and download and burn to CD the MS Blaster Patch for XP first.
After motherboard and hdds are set up, start computer with XP CD in, making sure in BIS settings, CDROM comes before hard disk in boot sequence or boot device priority. Press any key to boot from CD.
click here and select "How to Run a Repair Install" for screenshots. During repair installation, when asked for computer name, use old computer name (In existing XP, right click My Computer, Properties, network Identification tab to see the name). Otherwise you might be denied access to certain old files.
After repair, in XP, apply the Blaster patch immediately before going on the net.
Then you will have to reapply all Windows updates as the Repair brings everything back to CD state.