As Far as I know, Ubuntu use GRUB for their Boot Loader. I have it dual boot in my office's P.C. It's kinda complicated to create partitions and such things for a dual boot as Ubuntu doesn't really have user friendly Installation interface. However, for a single O.S, it's so simple. Do you dual booted your system ? Or is it the only O.S on the P.C ?

I see. If u can boot from the H/D from CD that means your installation is correct, the only problem is perhaps the GRUB wasn't installed correctly, or there was an error while installing it. Now what you have to do is just re-install the Grub. Here is what I think you should do : http://doc.gwos.org/index.php/Restore_Grub ( remember : DO NOT FORMAT or then it means installing it all over again ) then make sure that the GRUB is installed in MBR. Hopefully that helps.