That's right, it won't boot. Your chipset has changed when you swapped out the motherboard. You'll need that xp disk to boot and then do a repair installation of Xp from the boot menu. I'd try this approach first. This way you should retain all of your current software and registry settings. The only thing that may not make the jump ( I can't recall) is any XP service packs you've installed. If this is the case, you'll just have to re install them. Failing being able to do a repair, you're other option is to do a clean install of Xp by one of two routes. Deleting your current partitions, formatting and re install XP (you lose everything) or creating a new windows install directory, and booting from that location. You'll retain all your data from programs , but they won't start unless you re install them. One other thing you'll need to do is re validate your XP. You just did a major hardware change and as far as microsoft is concerned, this is a new PC. When you swapped out the motherboard, I hope you swapped out the power supply as well, if it was rated for just running a P3. The power requirements of a P4 jump dramatically and whilst you may have a late model P3 power supply with the right connectors, it may not be able to adequately handle the new load. I don't like to see anything less than a 450 watt PSU in a P4, although they will run with a 350. You never know if you're going to be adding more drives later or a vid card requiring extra juice.