Hi pjfurry - Welcome to w7forums
The first mistake, you should have installed XP first and then the newer operating system. Windows 7 is programed to handle XP and setup a dual boot menu. Windows XP will only screw up Windows 7 boot files.
What you may need to do is download, install, and use
EasyBCD to remove the Vista Bootloader. This feature is used for returning to Windows XP. Install XP to the second drive. Once you have XP installed you can then use Windows 7 install media to repair Windows 7 boot files.
If you do not have a Windows 7 install disk. You can download the
Windows 7 Recovery Disk. These disk will not install Windows 7 but will repair many things that maybe wrong.