It is a good idea to use two drives. I recommend that you download several iso files, version 12.04 (and 12.10 if you want to try it), flavours [vanilla or standard] Ubuntu, Lubuntu, Xubuntu, Kubuntu. Try them live before installation, and select what you prefer on that particular computer!
Then you can do as you write: Install without the Windows drive.
Check if Windows is booted via BIOS or UEFI. If BIOS, it will be straight-forward.
You need not go into the BIOS menus to change the boot order. Instead connect the Windows drive, keep the booting from the Ubuntu drive, and run this command
After reboot, you should be able to boot Ubuntu as well as Windows, selecting at the first text menu, the grub menu.
If UEFI, it is more complicated. Search for new threads in the Ubuntu Forums about dual booting of Ubuntu and Windows (7 or 8) where these methods are explained, for example this thread
http://ubuntuforums.org/showthread.php?t=2105622
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