It is possible to do a clean install of Windows Vista using a Windows Vista Upgrade CD, and therefore create a dual boot system with Windows XP and Windows Vista with the Upgrade disc. Though, it is a bit long winded. The following article guides you step by step:
Firstly you'll want to make sure that you have free space on your hard drive for the new Windows Vista partition. Next, perform the clean install with the upgrade disc using the instructions in the article above (making sure that you use the free space and not overwrite the Windows XP partition).