I would guess that either you don't have GRUB installed on the hard drive that is checked first, or you have problems with your GRUB config.
If during the inital stages of the boot it does not at any time mention that it is loading GRUB it is probably the first problem. The easiest and safest way to fix this would be to reinstall Ubuntu making sure to answer yes if it prompts you to install GRUB to the first hard drive.
If it is the second problem you will have to boot a liveCD and edit the file /boot/grub/menu.lst . There are a number of different things that could cause this problem, but two causes that I can think of would be a zero second timeout before selecting the default option or a hiddenmenu option. Reinstalling Ubuntu will get you a default menu.lst as well.
Hopefully something in that is helpful, if not maybe someone else has a suggestion.
BoysNightOut: I'm fairly certain that in this case ATA = IDE