Essentially what you end up doing is installing the Vista boot manager onto the XP drive. By default it will see the Vista drive, and by adding a new windows "legacy" drive, it will add the XP drive. (Make sure your XP drive is set as the first bootable hard drive in your BIOS). I can now dual boot flawlessly.
Another way is by partitioning a hard drive and installing XP to one of the partitions.
Then install Vista.
You then get an option to select OS you want at bootup.
Neat and very handy for those games not supported with vista.
i recently got vista ultimate and installed it. after finding some things wont run i decided to install XP Home, but didnt take out vista drive. after XP installed Vista drive will not load Vista. Any help would really help!