did you install grub to mbr?
whlie installing Ubunut, it asked me where to put the bootloader, and I selected the Sata HDD which was displayed as SDB. There were other options also in the menu one was written like "XP Windows NT Loader" which I think was BOOTLOADER of XP, so I selected a different location than that.
Not sure if it went to MBR or somewhere else. I was hoping that by default it should go to MBR.