Make a copy of %SystemRoot%\System32\Dhcp\Backup\Dhcpcfg and save it in your favorite safe spot.
Uninstall and reinstall DHCP. Before restarting DHCP, reapply your current Service Pack.
Restart the DHCP service to allow the installation to finish. Then use Control Panel / Services to stop the DHCP service. Edit:
Highlight the Configuration key and click restore from the Registry menu. Type the path of the saved Dhcpcfg file.
Restart the DHCP service. Verify your settings using the DHCP Manager.