With XP Home on the primary ide and formerly XP Pro on the second identical model 259gb drive here Vista simply refused to install to the second partition forcing the upgrade of Pro to Vista. Once that was done the boot.ini file for the Pro edition had to be edited to remove the planned reference to Vista on the second partition there.
If you don't want to have to go into the bios or if your bios has it the F8 boot menu to load 2000 you will need to in Vista to a second drive with the 2000 drive still plugged in. Vista's boot manager will then see the additional option added during the installlation when the installer detects all drives. Upon completion you will see the option to boot Vista or a previous version of Windows.
With the shareware known as VistabootPRO you can assign 2000 as the default version to load. You simply use one of the arrow keys to highlight Vista and press enter to see that load instead. Otherwise Vista will now be the default version that boots up. The new shareware spotted at
http://www.vistamania.org/index.php?option=com_wrapper&Itemid=47
(Gee? you can even rename Vista to "Bill's latest flop" if you want too!
Instead of seeing previous version of Windows you can put Windows 2000 with the rename option in the shareware.)