You installed Windows 7 as an upgrade (the default when one HD has an older Windows version and you're installing onto a different HD), ending up with a dual boot system, it can be fixed by reinstalling without the original HD plugged in, or with it in, though it is harder to set it up that way.
It is easier to just keep it the way it is, since it doesn't hurt anything, you can set the boot manager to automatically boot Windows 7 after a delay.