Firstly, do you have a different (from XP) partition or drive available for
Vista? Assuming you do, give your existing XP partition a name - for example
'XP' and the partition for Vista 'Vista'. This is because the drive letters
may get changed during the installation process, but not the names - it stops
you getting confused.
Secondly put your DVD in the drive and boot into your BIOS where you need to
ensure that the boot order looks for the DVD first. Continue from there and
the DVD will read and will ask you where yo want to instal Vista. Choose the
partition (or drive) you have ready for it and follow the instructions from
there. This is a 'Clean Install'. When it is finished (less than 1 hour on
most modern machines) you will have a choice during boot up between 'Vista'
or 'Legacy' . Legacy is XP.
Colin