Make sure that the motherboard jumpers are set to 1-2 pins. Go into BIOS to make sure all the hardware is properly detected. Preferably load default settings.
If you get past the cpu speed error message, see if it boots into Windows.
If it doesn't, or will only boot into Safe Mode, do a repair installation of your OS (is it XP?), installing over the top of existing Windows without formatting (keeping data and programs), which will then only require reinstalling all Windows updates. Unless the new motherboard happens to have a lot in common with the old one, you will need new hardware drivers for Windows, hence the repair reinstallation. (or you could try in Safe Mode uninstalling all motherboard related hardware drivers and reinstall new drivers - not sure how it will work).