I suggest not to touch dual boot. It`s all headaches. Disconnect the HDD that contains your current OS. Then install Windows 7 on the other HDD. The 2 systems will then be completely independent. Then reconnect the harddrive with the originall OS. When you power on, bring up the BIOS boot sequence (e.g. on an HP by tapping ESC) and set the boot sequence to the HDD where you installed Windows 7. Now it will boot Windows 7. If you do nothing, it willl boot your original OS.
The big advantage of that setup is that you need not handle the common bootrecord which can be a pain - e.g. when you uninstall one of the systems.
An aditional tip: Put all your user data (Documents, Pictures, Music, etc.) into an independent partition of your original OS. When you are in Windows 7, you just include all those files into your Windows 7 libraries. Thus any updates, regardles from which OS side, will be applied and your files are always up-to-date.