Agree that Cmos checksum error indicates a bad BIOS probbly due to flat battery
After changing battery and pressing f2 did you save and exit BIOS (f10)? Bios settings need saving.
recheck BIOS and make sure DVD drive is first boot followed by your hard drive (save and exit)
Make sure no usb drives or DVDs left in optical drive.
if still struggling disconnect none used drives (dvd and second HDD) and see if it will boot.