yeah, when you install windows XP you will get an option as to which partition to install it on, and also an option to create another partition, depending on how you want to do it, you can split you hard-drive down the middle and make 2 equal sized partitions etc...
install Windows XP on 1 partition. then once thats done, install Vista on the 2nd, make sure you dont overwrite the XP one... lol...
then every time you boot you should have an option for either "Windows Vista" or "Earlier Version Of Windows" (which is XP, Might show as "Windows XP"...)
Dave.