I have just installed a new P5G41T-M LX Series motherboard and two DDR3 x 2GB replacing DDR2. During startup a message says BOOTMGR is missing, press Ctrl Alt Delete to restart, on restart the same message is repeated.
You need to go into your BIOS Setup and make sure the HDD with your OS is set as your first boot device.
Also you might want to check the jumpers on your second HDD if its not a SATA HDD
Make sure your SATA data cables are in the correct places. First HDD (with OS) should be plugged into SATA port 0 on you Motherboard and 2nd HDD in the next available port.