NeoSmart Technologies had already released a nice Windows Vista boot loader modification tool named EasyBCD that works seamlessly in true multiboot environment. EasyBCD is the ultimate bootloader modification tool for Windows Vista that lets you boot from Linux as well as all other older versions of Windows in a multiboot machine!
Using EasyBCD is easy too. Just install it in a suitable directory either from within WindowXP or from Windows Vista. It works well in both environments. I recommend you to install it in such a directory that is accessible to all versions of Windows, and install it in both Windows Vista and Windows XP. Make sure the installation directory will be the same in both environments. This way, when you run EasyBCD in either environment, your EasyBCD configuration files will be the same, and you can change configuration from any environment.
You can set boot options, uninstall Windows Vista boot loader to recall Windows XP boot loader or reinstall Windows Vista boot loader through EasyBCD. EasyBCD also lets you install NeoGrub - a GRUB varient in your MBR so that you can boot your Linux directly from MBR.
The icing in the cake is - you can set your machine to boot in to BSD/Linux, or Mac OS too through this utility. Just give appropriate drive partition setting values in its Add-Remove entries, save the settings and you are done!
EasyBCD has detailed help and guide that you can access online. However, using EasyBCD is straight enough and does not require much skill.
Download EasyBCD from here.