Insert the Vista DVD and start the PC from that disc. Select Custom Install. Click on the drive where you got XP installed. Go to Drive Options and choose Format. After formatting is complete, XP is removed and Vista is going to be installed.
Before you do anything, what are the specks of your computer..You will need at least 1 gig of ram ... Installing Vista is not that easy, when installing on older computers.. it has a hard time finding all your hardware..