During Vista install you are given two choices: Normal and Custom.
The normal option will install over top of the previous OS using that partition.
The custom option allows you to select the partition/drive to which you wish to install giving a dual boot scenario.
As the OP has not once mentioned dual boot I would say it is safe to bet he went with the normal install option, thus rendering his prior XP install useless.
He has no option other than to format if he wishes to reinstall XP.