Yes, you can dual boot; however, as far as I know, both operating systems need to be on the same physical harddrive (but different partitions) - at least for Windows they do. Somebody correct me if I'm wrong on that. At least, that is how I have always done it.
Anyway, you can either use a program like
Partition Magic to set up your partitions and install different OS's on them, or you can create a partition yourself in xp and then when you install Vista, it asks you what partition you want to install it on, in which case you choose the one you created (or else it will install on top of xp).
Here are instructions for creating partitions yourself in case you don't know how:
Microsoft's instructions
Other instructions with pictures
After you have both OS's set up, your computer will ask you upon booting up what OS's you wish to load. In windows xp pro, you can access these settings in:
control panel - system - advanced tab - startup and recovery settings
Like I mentioned in the beginning, before you spend your time installing Vista, make sure you can actually have it on a different physical harddrive than xp. My understanding is that you cannot, but I could be wrong. Also, it is always good to back up data that is important to you in case something goes wrong and you lose your files. It has never happened to me, but I still back up just in case.
Hint: I highly recommend having the My Documents, My Pictures folders etc... in their own partition/drive, so that in case something goes wrong with the partition where the OS is installed, your files will remain untouched. However, for important data, always save onto a cd or dvd as an extra safety measure.
If you want, I can tell you how to set up your registry so that your computer knows that those special folders are now located in a different place other than in the C drive. That way you will still see them in the open and save dialog boxes etc... just like they normally are.