It's best to do a fresh install, all the old drivers can cause problems with the new drivers. If you can get it to work, you usually end up with a long boot time, and other random flakiness just because your registry becomes so bloated with needless entries from your previous build.
I have done this with windows 98 by going into the device manager right before you migrate the harddrive to the new system, and uninstall the drivers for everything relating to your motherboard like the CPU, chipset, drive controllers....pretty much everything in there. Start with the minor entries first, working your way up to the processor as the last thing. Then when you reboot with your new system you are going to get a long series of "new hardware detected" and a lot of reinstalling, requiring a lot of rebooting before you get through it all.
I don't know how well this will work with XP though, and I still say format and fresh install is the best route.