idea
You could try a repair install, but one method I'm sure works as we use it fro disk cloning at work, is as follows:
Plug HDs into P2 mobo
Bring up management console (r-click My Computer, select Manage)
Click "Device Manager"
Expand IDE ATA/ATAPI Controllers.
You should see an entry entitled "INtel blah blah" , "NVidia blah blah", or "VIA blah blah". Right Click and select Update driver, Select No not this time when it tries to connect to Windows update.
Choose INstall from from a list or location (Advanced) option
Choose Don't search. I will Choose.
In the next window should be an option for a standard Dual channel PCI, or some such thing. You're looking for the non-specific generic IDE driver. Select it.
If you had two entries in the IDE ATA/ATAPI section you will want to do both as they will probably have to change anyway. Also uninstall all your mobo specific drivers under System Devices in device Manager. Restart pc to save changes then shutdown and plugin p4 mobo then startup. You may have to start in Safe mode. You should be able to boot into windows now.
Bear in mind that your new mobo and old mobo will have to support compatible HALs. ie check under "Computer" tab in device manager and ensure your new mobo supports whatever is installed there.