More information is needed to get help and the best thing I am aware of is the bootinfoscript. Go to the link below, read the instructions, download and run the script and post the results.txt output file. It will give detailed information on your drives/partitions and boot files.
The problem I have is that I can only boot into either OS if I have a bootable disc in the DVD drive.
That would indicate you either have the CD set to first boot priority in the BIOS or you didn't install Ubuntu Grub bootloader to the mbr.