how do I get it to boot to XP instead of Vista without my input?

If you want to boot XP only, just insert you XP install disk, go to recovery console and use the 'fixboot' and fixmbr' commands.

If you want your bootloader to still have both options, but be on XP by default, open a command prompt with admin priveleges.
I'm not sure on the exact command, but I think it is along the lines of "bcdedit.exe /default {legacy}". I'm pretty sure there are many BCDEDIT guides which go into a lot more detail.

While in Vista, either - Control Panel>System>Advanced Settings>Advanced>Startup and Recovery, or with msconfig>Boot, or you can do it with VistaBootPro or EasyBCD.
http://www.vistabootpro.org
http://neosmart.net/dl.php?id=1