dozrmars made some points

yh! thats true , after installing WIndows XP, there's a tendency that Windows Vista might not show up during startup! this is true, but there are several articles with ways to dual boot Windows Vista/XP. You will need a small handy tool called "Easy BCD" - Google for that!
if u want it that way too and u would need to install the required drivers that u can use on XP.
My Dear Friend, when did an Operating System become a Device . . ?
First of all, What are Drivers?
I understand a driver to be a program that controls a device. Every device, whether it be a printer, disk drive, or keyboard, must have a driver program. Many drivers, such as the keyboard driver, come with the operating system. For other devices, you may need to load a new driver when you connect the device to your computer. In DOS systems, drivers are files with a.SYS extension. In Windows environments, drivers often have a.DRV extension.
It was not mentioned any where that an OS is a device, and drivers are required to have your OS show up on Grub
[oops, sorry - linux addict] 
errmm . . .on the Windows Boot Loader.
So Please, explain if I made a mistake somewhr!