|You don't say which versiuon of XP you have installed.|
XP home can be upgraded from the XP desktop by inserting the DVD, if autorun is disabled, you will need to double click on the install file on the Vista DVD.
PLEASE NOTE a DVD Rom is required!
If you have XP Professional a clean instalation will be required.
This means you will need to save your files and boot with the Vista DVD making sure bios is set to boot from cd/dvd first.