You have to modify the boot.ini file.
I did it once when I dual-booted Vista and 7....
Save your boot.ini on notepad so that if you muck up you have a working copy.
Set the time to choose between operating systems to 0.
Then you have to delete the string which refers to booting the OS from the other hdd.
Sorry if it doesn't sound clear, it's difficult to explain if I don't have the pc in front of me lol.