How can i choose which operating system i want to run at start up i have Vista and XP installed?

I actually have two seperate hard drives installed...my new pc came with windows vista...i took the hard drive out of my old pc and inserted it into the new pc and uploaded all of the drivers...but when i shut down the pc and restarted it's only loading the XP hard drive..what can i do so that when is tart up my pc i have a choice to chooce between XP and Vista?