If it's an IDE drive, and a second drive, you set the jumpers so that it's a slave and connect it to the same cable as the first drive (or a second IDE controller on the mobo if available), and it won't boot.
Now you usually have to change the jumpers on the first drive, depending on the mfr, so that it is jumpered as a master instead of a single drive.
That done, when you power up, the slave won't boot.
I just bought a new computer, but I want to add my old hard drive to the new computer. I can change the jumpers and stuff, but can you please tell me how to not let the old hard drive boot up xp, because it used to be the master drive in the old computer, and it has a recover partition built in with it.