Welcome to NST Highmaster.
Here's the gen from Vista Help
Which drive letter will your clean installation of Windows Vista use? That depends on how you install it. If you currently have a working copy of any Windows version on drive C and you install a clean copy of Windows, drive letters are assigned using the following logic:
•If you begin the installation process by booting from the Windows Vista media and choose a partition other than the one containing your current copy of Windows, the new installation uses the drive letter C when you start up. The volume that contains the other Windows installation uses the next available drive letter. When you choose the previous Windows installation from the startup menu, it uses the drive letter C, and your new Windows Vista installation is assigned the next available drive letter. In this configuration, you can be certain that your current operating system is always on the C drive, but drive letters assigned to volumes you use for data may shift in unexpected ways.
•If you begin the installation process by running Setup from within your current version of Windows and use the Custom (Advanced) option to perform a clean install on a partition other than the one currently in use, the new installation uses the next available drive letter. The volumes containing each installation have the same drive letters regardless of which Windows version you select at startup.
There’s no inherent reason to prefer either of these options over the other. If you prefer the consistency of knowing that all system files and program files are on the C drive, you’ll probably want to choose the first option. If you would rather use drive letters to keep track of which Windows version is running at any given time, you’ll prefer the second option. But either configuration should work reliably with any combination of software, hardware, and settings.
Now that XP has installed that way (like my system), you can't change anything with boot options.
There will be a problem if XP can see a C:\ disk, because some software will insist on putting stuff on C if there's one there.
You can use HnS (Vista Hide 'n Seek) to hide Vista from XP, which you should be doing on all of your dual-boots if you use system restore. (XP destroys Vistas restore points if if can see them)