Um.. what?
First, i would recommend backing up your data before you do this, just in case something breaks.
In order to dual boot, you will have to have either, 2+ partitions on your hard drive, or, more than one drive. You have XP installed to your hard drive, and it is called C: But you need to have either (as I said) another partition (D: perhaps?) or another hard drive altogether to install Vista to.
If you do not have either of those, but have lots of free space, then I recommend running PartitionMagic to give Vista at least 30GB of space, how much you actually give it though is dependant upon what you want to do with it.
Once you have a second partition/drive and have backed up data. Stick the Vista disk in the drive, boot to it, and the choose the partition you want to install it to.
Once it is done, it should work just fine.